If its an online guide, I find I usually have to cross reference between tfu.info and transformerland. I find that both sites tend to miss certain things about figures part wise and such. Plus, lots of the accessories are very difficult to identify, and the different photo view points the two sites give you really help when you're trying to ID parts. If you're wanting to carry around a book with you, I've found this one is pretty nice, even just to read. Amazon.com: Transformers: Identification and Price Guide (9780896894457): Mark Bellomo: Books I'd stay away from the Warman's guide. Its not that good.
I have the Bellomo book and I really like it, but I think the OP is after something for post-G1 identifications.
I want to say those old Cybertronian Unofficial Recognition Guides started into Beast Wars but I'm pretty sure they never finished. Other than that, I can't think of anything. But I too would love to have an actual book with photos of the Movie and Unicron Trilogy lines and the other stuff. Holding back publication of such a book would likely be the fact that these lines aren't vintage yet and therefore may not be seen as collectible by professional collectors, which would limit the book's appeal. Also, publishers like to publicize these types of books as Price Guides, and it may be too early to set standard, non-wildly-fluctuating prices for these toys. (I seem to recall Bellomo or one of the other authors, Alvarez maybe, stating that they hadn't wanted to include prices in their books but the publisher insisted.)
While a book can be handy, it can't be easily updated/revised, so perhaps the online guides such as tfu.info, cobraislandtoys, or transformerland would be your best bet.
