As with many other subjects, Wikipedia is actually pretty good for a basic overview -- with the usual caveat that sources must be checked and double-checked. If you're interested, however, here is an extensive but incomplete list of the other resources I referenced when writing this series. (first posted 9/21/2012, last updated 3/18/2013)
Chinese Text Project: The best source, hands down, for accessing original historical texts.
CHINAKNOWLEDGE: This is the only general history site I'm going to recommend, as it's fairly thorough, and more importantly, meticulously sourced.
Lin Yutang's Chinese-English Dictionary of Modern Usage: My favorite Chinese-English dictionary, though it's a bit less useful when it comes to classical texts. But that's where ctext.org's built in dictionary comes in handy.
Dialects of China: Another good dictionary, but harder to use. (Wiktionary is a decent alternative.)
unnamed Chinese database: Dictionary with theoretical reconstructions of Old/Classic/Middle Chinese.
Sino-Platonic Papers: Interesting and off-beat body of English language research. I love this series. Really fascinating stuff here.
Warring States Project: Exactly what it says on the tin. An archive of Warring States research in English, hosted by the University of Massachusetts Amherst. I should warn that they don't use any of the standard romanization systems, however, and the material here is a bit more esoteric.
Chinese Text Initiative: Another great archive of texts (translations included) hosted by the University of Virginia, with more of a focus on literature. If you want poetry, this is the place to go. (I am actually not exactly enamored with most existing poetry translations in English, but it's better than nothing!)
Traditions of Exemplary Women: Also hosted by the University of Virginia. (I am unclear about what is going on with the organization of these archives, but as of current time of writing these are the two links I used.) The main purpose of this archive is the study of the Lienu Zhuan, but it also includes plenty of other relevant bilingual texts, including the Zuo Zhuan, Bamboo Annals, Book of Rites, Book of Odes, and Book of Documents, plus various Confucian texts.
Wengu Zhixin: More poetry/texts. This site features both the original Chinese as well as English and French translations, and even one German translation of the Dao De Jing. (See above comment on English translations.)
JSTOR: Once upon a time, I had free access to JSTOR. If you're at a participating institution, this is an invaluable resource for a vast variety of topics.
Persee: Like JSTOR, this seems to be an online archive of various academic journals. I've stumbled across a few useful academic papers here, but haven't done more in-depth searching.
The following list assumes a basic grounding in Chinese history/culture. I have no recommendations for general overview history books. And I've seen a few decent-to-good mythology/folk tale collections floating around, but when it comes to oral tradition, I think asking people from the actual culture is more interesting or productive, unless you're looking for something particularly obscure.
Allan, Sarah. The Shape of the Turtle: Myth, Art, and Cosmos in Early China. State University of New York Press, 1991.
This book is a bit dated, and the theories have since been questioned, but the underlying research is nonetheless solid. Definitely a good launching point for further investigations. Allan's other work is also worth a look for anyone interested in the Shang/Zhou.
Cook, Constance A., and John S. Major, ed. Defining Chu: Image and Reality in Ancient China. University of Hawaii Press, 2004.
The definitive book on Chu culture. Now if only someone would put together books on the other states...
Sage, Steven F. Ancient Sichuan and the Unification of China. State University of New York Press, 1992.
Another excellent book on one of the "frontier" states/territories during the Zhou.
There are a couple of translations I haven't had a chance to really explore yet at the time of this writing. I also referenced a few Chinese-language books but have no particular recommendations, as I didn't look at anything really academic or rigorous but rather used them for the much better illustrations/photo references, and to double-check minor details. The following is a particularly nice one, however:
Liu, Yonghua. Ancient Chinese Armor. Shanghai Guji, 2003.
In simplified Chinese, that's 中国古代军戎服饰 by 刘永华. It's illustrated!
Hawkes, David, trans. The Songs of the South: An Ancient Chinese Anthology of Poems by Qu Yuan and Other Poets. Penguin Classics, 2011.
This is the reissue of a gorgeous translation of the Chu Ci (楚辭) originally published in 1985 -- notable in particular for its thorough, extensive commentary, which not only lays out the context for these poems but also explains various theories and variations in interpretation. For the record, I much prefer Hawkes's interpretations to some of the more traditional arguments I've seen out there (and which he outlines in his commentaries). With that in mind, though, it's worth noting that even if one disregards the infamous ambiguity of classical Chinese, there's a great deal that has been lost/forgotten/edited/recontextualized over the years (i.e. a lot of filters to work through), or that we have simply yet to uncover archaeologically, and therefore a ton of room for differing interpretations. Or in other words, if you're interested, it's worth familiarizing yourself with all the major schools of thought on this anthology, though this is the only book I'll be including on my list (because it's my favorite of the various translations I've looked at, and the main interpretation I've chosen to work with/influence my worldbuilding).
There's only one I can think of in English at time of writing. (Though I remember a few good ones on the Great Wall and the Silk Road.) I'd recommend documentaries as starting points only, AND recommend watching them produced by different countries in order to get different perspectives. Even the best ones I've seen are very shallow treatments of the subject matter. They do help with visualization though.
I love museums. Unabashedly. The one with the most extensive/nicely organized collection of Chinese material is arguably the National Palace Museum of Taipei (the corresponding Palace Museum in Beijing is rather trickier to tour and very Qing-centric for obvious reasons), but there are smaller ones out there too with somewhat more obscure or localized tidbits -- and a couple of museums have some material available online. Not much, though, and you do need pretty solid grounding in the historical context to get the most out of museum visits.
To be honest, you're not going to get a lot of accuracy from this stuff, but it's sure fun to consume anyway. Since this is a resource list though, I'm going to try to limit it to examples that at least make an attempt to be accurate, or examples that are otherwise interesting/noteworthy.
Confucius (2010): A boring but rather lushly realized and relatively well-researched biopic that showcases many of the cultural aspects of the time.
The Great Revival (2007): There have been multiple televised takes on the Wu-Yue conflict (upon which Book III of my series is based) -- I can think of three just in the last decade... all within the space of a single year. Of all the recent versions, I suspect the best is this one. It's occasionally boring (some of the subplots are rather inane or even infuriating), visually uninspired (and inaccurate!), but very Shakespearean in structure and excellently acted for the most part. The other two dramas may be worth checking out for the sake of comparison though, as each one has vastly different interpretations. The Conquest (2006), for instance, puts much more emphasis on melodrama and romance. King Goujian of Yue (2007) puts more emphasis on cheesy, over-the-top action scenes. Pick your poison.
Hero (2002): Not even the slightest bit accurate, but I feel obligated to mention this as it's probably one of the movies Western audiences are most familiar with. It's basically a flashy but gorgeous/entertaining and completely fictionalized take on the Qin Emperor and the various asassination attempts made on him... and is a movie that makes much more sense thematically if you understand the earlier Warring States context. Fun fact -- Chen Daoming, who plays the emperor in this movie, is the same guy who plays Goujian in The Great Revival.
"Sword of the Yue Maiden" by Louis Cha/Jin Yong: Accuracy? What accuracy? This is wuxia, pure and simple, by the master himself. Not his strongest work in my opinion, but still a quick, entertaining read. The story is based on an actual, albeit somewhat obscure episode recorded in the Wu-Yue Annals, and worth reading for that alone. I've seen at least one fun comic adaptation of it floating around out there too.
"Longing in Silence" by Tan Dun: A track from The Banquet (2006) OST (there are two variations, one sung by Zhou Xun, one by Teng Ge'er). Originally "The Song of the Yue Boatman", an obscure but fascinating verse transcribed and translated in the Shuoyuan. Tan Dun took the translation and set it to a gorgeously haunting melody for a movie that does NOT take place in the relevant time period. Oh well. Beautiful tracks anyway.
Kingdom: A seinen manga/anime action series chronicling the Qin Emperor's rise to power. For an action series (which typically prioritizes the Rule of Cool over realism and historical accuracy), this is actually pretty well researched. If you're willing to overlook over-the-top action scenes and mediocre (but competent) art/animation, it's quite entertaining and the characters are engaging. Funimation did an excellent translation of the first season that can be streamed here.