Assuming that
this database site is complete, there are around 550 books. 700 can only be obtained if the guides and unofficial books are counted as well.
Still, if the official novels are now all available as ebooks, it's worth looking into
edit: or maybe not. These books cost between €8 and €15 (!), at least in The Netherlands and at Kobo Netherlands, and that's true even for the huge backlog. That's too much for books like these. I only pay prices like that (especially as high as $15) if they fall into category "Iconic Must-have Books". Books in a shared universes (FR, Eberron, Ravenloft, etc...) are not worth more than about €5-6 to me. I bought all my FR books during the Kobo sprees.