High-grade collectibles are known to be more likely to be Unique mob drops. Except for the items you make or drop items (Boss Monsters).
In Lost Ark, collectibles of all grades are dropped from all level mobs In that area. But my experience, High-grade collectibles are dropped from regular mobs too. So it's better to kill as many mobs as possible. If possible, I think it's a good place to have a regular mob, a Unique mob, or an Elite mob together.
I farmed all the Lost Ark collectibles for the first book and so far:
Unique monsters and elite ones (yellow border) have way higher chances for a collectible to drop.
Density seems to affect collectible chance also (or maybe it’s just because you killed multiple at the same time so it feels like that).
Sometimes I got a really rare collectible from a simple white mob roaming around = don’t underestimate them.
Dungeons don’t seem to be worth it. They’re long and mobility is really restrained. Maybe in higher areas where dungeons have a lot of elite and unique monsters it might change, but on the first ones, it’s not (I farmed the first dungeon and get a lot of
Lost Ark Gold).
My advice: Kill everything that you see, but try to find a ? route ? that allows you to farm unique and elite monsters over and over. Their respawn timer is really fast so pick a route that isn’t too long (or too fast too) so you can kill them as much as possible.
A good way to do this is to pick an area for your current book and look at its map on Papunika. It will show you unique monsters and by visiting you’ll see how many elite are around it. If you can do a circle just do that over and over.
The best part is that when you’re done collecting some of them in a book (like for example the blue collectible Aguiloss’ Skin in the first one) you can still get others, and they sell REALLY well!! I would sell Aguiloss’ Skin for 85 gold each and it was pretty much instant (I don’t think people like to farm lol).
Most importantly: Have fun!! If it gets boring farming over and over just stop and do something else!
Lost Ark: How to get Adventure's Tome collectibles 
