All types of it: novels, stories, short stories, guides, collections, info-brochures, instructions, compendiums, etc., etc. - the more, the better ! And especially books which are connected to cultures, locations, history and fauna / flora of the game itself. ... And a great (secret) library to discover ancient myth and such ! B)
Edit: but I don't think this is really an issue here, since Beth has proven always of the right ideas / intentions for quantity, quality and variety regarding those things in previous games, imo.