A proof, or another term is advance reader copy (ARC), or advance galley proof, is usually a trade-paperback sized softcover edition of a book that is sent to reviewers, etc. before the actual publication of that book.

Because these are generally issued in very small numbers and many are actually used for what they were distributed, they are usually fairly hard to find. The largest edition of a King proof that I know of was the US Rose Madder--10,000 copies--where the actual Rose Madder US edition probably had a first edition first printing of more than a million copies.

These volumes represent the actual first printing of the book. One of the major reasons I like getting them is that, especially in the case of my getting the proof before the book is published, I enjoy reading the story before most other people.

There is no single resource that I know of that catalogues these. That's what I'm trying to do. Accurate information on these is usually VERY hard to get. For example, I emailed the UK publisher of Lisey's Story about their proofs. I got a response saying that all their proofs had been numbered, but one that I'm sure of made it out without a number. I've seen the pictures!

Hope this helps!

John