I don't mark up books that I feel are collectable or sacrosanct. Obvious candidates are known limited printings, special editions, and other items marketed as "going once." But Super Pony, how can you tell if a regular book is going to be collectible or precious at the outset!? Are you a wizard!? What about the Book of Erotic Fantasy?! No my dears...I am no clairvoyant magician. I am simply one of the reasons that game books become collectable and people make decent money selling said books. I am a full fledged member of the "Presently Destructive Order of Nostalgic Hindsight Book Protectors, Inc." It's a gnomish guild...
Basically this means my library goes through the /facepalm cycle of:
* Buy new shiney book and use it brother...I mean... annotations, throw it at people, wrestle it away from the dog, dogear pages, show up too late to keep my child from "coloring" it, overuse and overstress the spine, leave it in a humid bathroom, inflict it with taped together pages, etc. (This process takes a couple years but typically books I use regularly get battle scars).
* Retire/Gift/Recycle the wounded soldier (usually when a new game I like comes out, a new edition or whatever reason causes me to move along to new games)
* Get bit by the nostalgia bug whilst remeniscing about the 'good times' we had in that old game. I hit my local game stores (Portland is blessed to have several good ones with extensive 'archives') and only occaisionally browse the interwebs.
* I then locate a good or better condition copy of the book (if I can locate a copy in the same print run as the original all the better). I use my powers of financial obfuscation (also known as saving my discretionary monies...yes I'm in my thirties and have an allowance). Then I acquire the object of my desire.
* Finally this rebirthed tome is carefully placed in my library in the study where it twinkles like a gem and is only occaisionally called into active duty.