Depends on what you want, but I still think Book of the Righteous is the best D20 book yet. In addition to a great explanation of how gods/pantheons work, the Holy Warrior section is worth most of the cost.
Before buying the Witches Handbook from Green Ronin, download the Netbook of Witches (free). It is quite extensive. Indeed, unless you are averse to pdfs, I'd buy 3 or 4 pdfs instead of one D20 book. You can usually find a specialized topic or 5 you are interested in for the price of one printed book. Of course, there is the whole print issue.
For printed books, I'd recommend any Green Ronin product, they are are very consistent in terms of quality and usefulness. Given the first 2 books in the series, I'd wait for their Book of Fiends to come out, and buy that.