D'oh! Just seeing this now. Seconding the recommendation of Pulleyblank -- it's more of a reference book than a textbook, but it's an absolutely fantastic resource for anybody setting out to learn classical Chinese.

Most Chinese students use the 古代漢語 series edited by Wang Li 王力. This should be easy to find in pretty much any bookstore if you're in China, and it's a good introduction as long as you don't mind reading long passages in traditional characters. (Or maybe there's a 简体 version around these days. My copy is 繁體-only, but it's an old edition.) On the other hand, it's intended for native speakers of Chinese, so it might not be the most gentle introduction.

- I began studying Classical Chinese with A First Course in Literary Chinese by Harold Shadick. It worked for me, but more recent textbooks will probably do a better job of explaining things. It also uses Wade-Giles, which some people find annoying. (I'm not one of them.)

- I recently looked over A Practical Primer of Literary Chinese by Paul F. Rouzer and thought it looked promising. Rouzer's textbook assumes that students will not necessarily know Mandarin -- this is a good approach, and one I'd love to see more of...

- ...but you might prefer Classical Chinese Primer by John C.Y. Wang et al, which occasionally saves itself and the reader time by explaining things by analogy to modern Chinese -- 矣 behaving similarly to modern 了 and that sort of thing.

- BLCU has also got a series of classical Chinese textbooks aimed at foreign students. I flipped through them in a bookstore once like eight years ago, so can't say for sure whether or not they're any good, but BLCU's teaching materials are usually less bad than average.