Okay so, I know this is a stretch and will probably never happen, but here are my thoughts.
There is a lot of content for RPGS out there scattered through multitude books, which makes playing the tailor-made campaign you want a really expensive endeavor. Not every one needs every monster, or the information on every country. Adding the ability to buy single chapters (perhaps counting those as part of payment for the whole book) would help financially a whole lot.
Personally, I am really curious about the Geb chapter of the Book of the Dead, but buying a whole $30+ book for a single chapter is not really worth it.