Illustrations? Or just text?
First editions? Or you don’t care?
Signed by the author? Or not?
Most of these things don't really matter to me. If it is a favorite book I like to get it in a hardback, because of it's durability. I have never been to an author signing, but it would be fun to do sometime. I have one or two books that were signed, but it doesn't really make a difference to me. The only kind of books I really am "collecting" are classics. I try to find them in hardbacks, but it doesn't matter to me if they have illustrations, if they are a first edition, or if they are signed. As long as I can read the book, I'm happy. :)