What good textbooks are there for self-studying Japanese? I can read hiragana and katakana fluently, but will need help with the Kanji.

Konnichiwa,

I have just found this site - it's free and I think it's well organized.
http://www.studyjapanese.org/

This is NHK online japanese lessons - it's also free and there're downloadable PDF files and audio files.
http://www.nhk.or.jp/lesson/english/index.html

Have fun!

essential kanji is a good course
japanese grammar by yuki johnson is a good book
borders has a good japanese course set that is like $25

a book called, " Genki" for begginers. Also useful when you have japanese classes. You ca look it up in google books. It's gonna show. It costs around 30-40 for the textbook. For the workbook, around 17-20 dollars.

I can recommend none more than Minna no Nihongo, especially as you can read Hirgana and Katakana. Its not so good for self study though since there is no English. Genki is a fairly good one, Romaji drops out pretty quick from that which is what you need in order to learn properly.

The Japanese for Busy People is a good series - available in romaji or in Japanese script appropriate to your level.

There's also a great podcast website with material for all levels. I've included the link below.