I've written a number of books.
There is a trickle of sales after the initial post-publication burst. That trickle can go on seemingly forever. I'm still receiving royalties - and sales - for the PDF version of a book I co-wrote about 15 years ago. Nothing beats the advance-busting first month for GURPS Martial Arts, but it still sells, in PDF and POD (and it's even on sale, in PDF.) I understand this to be the "long tail."
Out of all of my books, the most steady trickle of sales seems to be monster books. Every month I get my SJG royalty statement. My two monster books - DFM1 and DFM3 - sell steadily. They don't often sell a lot - usually low-mid single digits - but I can't find a statement in my archives that has a 0 for either of them. Some months, no one wants to read about Ninja or Gladiators. But someone always wants monsters.
I get this, as a GM. Looking at my book shelf, I have:
AD&D Monster Manual
AD&D Fiend Folio
AD&D Monster Manual II
D&D Monster Manual (3.0, I think)
D&D Monster Manual (5e)
Pathfinder Bestiary I
Pathfinder Bestiary II
Pathfinder Bestiary III
Rolemaster Creatures & Treasures
Rolemaster Creatures & Monsters
D&D Monster Cyclopedia
Munchkin Monster Manual
The Basic Fantasy Field Guide of Creatures Malevolent and Benign
Swords & Wizardry Monsters Book
. . . and those are just the monsters-only or monsters-mainly books that aren't GURPS. I've got at least six GURPS books that are mostly monsters, too, not counting PDFs.
I don't even play most of those systems.
I'd buy more if I had room or saw a cool enough monster book.
So I get this avarice for more monsters. You can always use inspiration even if you can't use direct stats.
I'm therefore always keeping an eye out for an opportunity to write more monsters, especially if I can get a royalty-producing book out of it. They seem to make for the most steady sales over time.
I'm curious if that's true for others - if you write for games, do your monster books provide the most continued profit over time aside from any "core" system books? And everyone else, do you find yourself "needing" more monster books even for systems you don't generally play?