Mumford's three part series Tata Lectures on Theta is well worth reading. (I wish I had read them already...I could sure use the information they contain.)
Added: Or really, close your eyes and pick a book by Fulton, Hartshorne, Kollar, Mumford, Silverman....If you get the book by Hartshorne that you've already read, pick again. Otherwise, whatever you picked will be a fine choice.