In Feuerberg, I got rid of knowledge skills and added books to the gear list instead. If you carried a book (one of six to nine gear slots you might have), you could read it as you went and ask questions about the topic, and there was a chance (usually on a 4+ or 5+ 6) that it would answer the question. If you wanted to play a smart character who knew a lot of stuff it was easy enough - just carry a lot of books around with you. In theory it took a turn of reading to answer any question, though I was sometimes a bit flexible about this.
I liked this system a lot because it turned knowledge into a scarce commodity by tying it into two of the existing subsystems that govern scarcity (the marketplace of gear, and encumbrance). It also allowed encouraged PCs to plan ahead about what topics they thought might be relevant, while giving them flexibility about what they could know, instead of investing a ton of skill points or training into knowledges that might not turn out to be useful. I think most of its faults in practice (which were few) were the result of me not being consistent or investing enough time in producing possible book suggestions on my end.
One of the meta-game structures of Into the Depths is that instead of a ton of powers from magic, or your species and class, or some other intrinsic aspect of you, most of your "powers" are either obtained or enhanced by gear. The idea is that you explore a dungeon or wilderness area using your gear until you reach a set of obstacles that you can't overcome with your current gear, then go back to your home base, change out your gear load, rest up, and go on with the expedition until you hit another set of obstacles you needed new gear for, etc. I tried in Feuerberg, not always successfully, to often have treasure apparent but requiring special gear to extract. e.g. a fossil embedded in a boulder that would be extremely valuable but requiring you to bring along special tools to cut it out without damaging it.
Books as gear are meant to play into this cycle. You encounter some incomprehensible gibberish in a long dead language no one speaks - get a book on the subject and decipher it. You want to know what kinds of monsters are roaming around (i.e. are on the wandering monster tables)? Get a book on the subject and read it. You want to build a fortress? Better read a book or two on architecture.
To enhance this in future games of Into the Depths, there are three changes I'd make to the initial idea. The first is to simply add more books covering doing more stuff. Cracking codes, deciphering languages, explaining how to build complex mechanical devices like traps or certain machines, etc. This is in addition to books that just straight up answer questions on archaeology or geology or whatever.
The second change is to introduce expendability to books. I like the idea of a usage die but I think it'll be complicated to track, so I'm just going to have each book capable of answering 1d6 questions on a given topic before it's exhausted. Once it's gone, you have to buy a different book, even if you want more information on the same topic. This helps prevent PCs from sitting around asking infinite questions while they're on the expedition and have the book in their possession, as well as effectively dealing with the question of what they can do with the books during their downtime (they can exhaust all of the questions a given book can answer, which is what they were going to try to do anyhow). Rare books you get as treasure might allow for more questions.
The third change is to introduce differences in quality. This will take two forms. The first is whether the books allow you to a 5+ or a 4+ on a d6, with better books (more expensive or harder to find) allowing success on the lower rolls. The second is that basic books cover one topic, but better books can cover multiple topics. This means you can haul along more knowledge without more encumbrance.
Some book ideas (all work on a result of 5+ on 1d6):
Cryptography manual - Decipher codes you encounter
Phrasebook - Speak a language you don't know
Grimtooth's Traps - Design and build an overly elaborate trap
Farmer's Almanac - Predict the next day's weather
Code of Law - Bullshit your way out of legal troubles
Bestiary - Fill in boxes on the wandering monster table ahead of time
Collector's Catalogue - Appraise the value of non-monetary treasures
Herbarium - Identify helpful and dangerous plants you encounter