James Gasik
We don't talk about Pun-Pun
DM books don't sell, which is why WotC doesn't make a lot of them. That's why almost every book they do make has player facing content as well.I think the game would benefit from 2 DM-focused books:
1) Stuff to throw at my players' faces: mix the MM (and a monster builder) with the encounter builder and evolving traps from Xanathar's, NPC reaction rules, puzzles, journey rules and magical terrain hazard from Tasha's and Saltmarsh.
2) The art of DMing: worldbuilding stuff, alternate rules, magic items (and a magic item builder) etc
After all, for every DM, there's a handful of players who want fun new options for their characters. Why market a book for a fraction of your base?