That's part of a quote from Mickey Spillane. Here's the full thing: “Nobody reads a mystery to get to the middle. They read it to get to the end. If it’s a letdown, they won’t buy anymore. The first page sells that book. The last page sells your next book.”
At Maine Crime Writers today I talk about last pages and last lines. Nailing the ending is one of the hardest parts of writing a novel. As I write in the blog, The Poacher's Son didn't originally end where it now does. And it took me a number of drafts to find the right note on which to conclude Trespasser. I knew the last line of Bad Little Falls early on.
I guess the test will be if it helps sell my next book, right?