You can look at is as having three books increases the buy-in for GMs. Or, you can look at is as having one book increases the buy-in for players.
As a player, I don't want to have to carry around a book three times the size, full of monster and GM information that I don't really need. So, I'd want at least two books - Player's guide and DM's guide.
If you want the DM to buy just one book, have the Player's guide be the first section of that book. Have it be the same, in layout and page numbers, so players and GMs have the same reference material, and can talk about page numbers and how far down a column you need to go to find a particular rules-detail.
As a GM, I'm happy to have three books - D&D has so many monsters that it calls for a separate book for them.
As a player, I don't want to have to carry around a book three times the size, full of monster and GM information that I don't really need. So, I'd want at least two books - Player's guide and DM's guide.
If you want the DM to buy just one book, have the Player's guide be the first section of that book. Have it be the same, in layout and page numbers, so players and GMs have the same reference material, and can talk about page numbers and how far down a column you need to go to find a particular rules-detail.
As a GM, I'm happy to have three books - D&D has so many monsters that it calls for a separate book for them.