Passive book burning is an interesting concept, but I think it's also just entirely unrelated to active book burning. Whether Trog should lose the ability to throw books and make impromptu flame clouds, and whether worshiping Trog should mean permanently sacrificing some spell availability even after you abandon him/her, are completely different questions. You don't need to replace active burning with anything if it isn't serving an important purpose (and in my opinion, it isn't), and you can add book burning to Trog's wrath even if you keep active burning too if you really want to. Personally, I'm against adding passive burning - while nerfing Trog isn't always a bad thing, I think it's good to have Trog->TSO as a straightforward way for players to get their first 15-rune win, and in general I'm against gods besides Ru having permanent wrath.
Meanwhile, active book burning should be scrapped even if books aren't removed as inventory items. It's a clunky concept that only exists because of flavor, and while the flavor is fantastic, if you look at it from a gameplay standpoint it blatantly goes against DCSS's design and should have been removed ages ago.
Consider the two purposes book burning serves:
- It's a form of item sacrifice. Item sacrifice was already removed from every other god in the game for being too tedious, and other gods only required one button press to sacrifice an item while Trog requires two (admittedly, you also encounter book way less than you encountered weapons on old Elyvion or items you weren't using on old Nemelex).
- It's a variant of Conjure flame that can target enemy tiles, but takes two auts, two extra button presses, and uses consumable items that take up one inventory slot each. Trog really doesn't need this extra power, since he'd still be one of the best gods in the game without it, and if for some reason Trog having a very limited conjure flame is a good idea, it really shouldn't be so ridiculously clunky.
Basically, from a flavour standpoint, book burning is an awesome mechanic. From a gameplay standpoint, it's a terrible, clunky mess that should have been removed years ago. Since DCSS's current design philosophy strongly prioritizes gameplay over flavor, active book burning should go.
TL;DR: Active burning is a bad gameplay mechanic and should go, passive burning is completely unrelated but I personally don't like it.