If you’re curious what publishers are talking about when they reference the “bluelines” of a book, these are the bluelines for It: The 25th Anniversary Special Limited Edition by Stephen King, which Cemetery Dance Publications will be publishing later this month:
This is the official approval copy the publisher gets from the printer and, as you can see, these days they aren’t blue, but they serve the same purpose. This stack of loose papers represents the way the text will be presented on the pages of the book. You look the pages over to make sure nothing got lost in translation between when you finalized the design and when the printer loaded everything up on their end. The pages are printed in black and white, even if there are color elements to the page design. Color artwork that will be tipped into the book is also not included, although you are sent separate proofs for them.
For more complicated projects, you have the option of asking for a “folded and gathered” proof. This is done after you approve the bluelines. The F&G is printed on the same presses that will be printing your actual book, so to create the F&G, one copy of the book is basically run off the press all by itself. The F&G includes everything the final book will: all color elements, the artwork that will be tipped in, etc. Here is what the F&G for It by Stephen King looks like:
Those are the real pages and the real artwork, printed on the real paper stock that will be used for the book. The pages are grouped in their signatures of 8, 16, or 32 pages, but they are not yet bound together.
So the F&G is essentially the final book, just without a binding. It is the absolute last chance to fix any problems.