This is a pretty easy alternate question to answer for me. I did not want to answer the original one about a PWYW publisher because it does not really invite discussion. Plus my finger has been off the pulse for too long and I may do someone a disservice. What makes a particular one of my RPG books special to me over another? Well, that I can sink my teeth into, although the answer is quite simple.
Uniqueness
In one word that is it. How they are unique may vary but the feel of a unique book makes it special to me. I mentioned a couple of days ago that the Edge Of Space books that I own in hard copy are very special to me. Among my favourite books in the collection. Why? Well, because they were hand crafted and printed by the author of the game and are so very unique. I know others exist but I have never seen them. These books are super rare and super special to me!

Other books that I own and find very special? I have two hardback Lords of Gossamer and Shadow rule books, which is special enough. One of those is signed by the author, Jason Durall – which is awesome. I read and reread these a lot. Great game. I also have a Cypher Systems rule book with a book plate in it signed by Monte Cook which is also pretty cool. My black faux leather covered copy of Dungeon Crawl Classics also ranks right up there too!
Anything else special?
I have a limited edition Era the Consortium rulebook that was individually numbered. I have a copy of the Hunter Planet Australian RPG from the 80’s that is super rare and hard to find. I have quite a collection of the James Bond RPG by Victory Games that is also a bit of a collectors item. Finally, I have a complete and near mint boxed edition of Tank Girl RPG that I will never play but is super special to me. I also realise now that my answer on Day 17 is probably incorrect. It would have to be Tank Girl. It is special to me for sentimental reasons though. I got it as payment for helping a friend move a large amount of gaming stuff from another store to his. I love Tank Girl through and through.
I have bunches of other books as well but the above are those that I cherish the most. To make it special it has to be something rare and/or unique. I love RPG’s and seriously crawl through any lot of books I can to see which will next make my collection. Keep rolling!