Depending on your publisher, it's an author's job to find endorsements for his/her book. It can be tough. First of all, you want to have someone endorse your book so other people will read it and say, "Ah. If so-and-so liked it, I might too." Not everyone is going to read the endorsements on a book, but I believe they do make a difference.
I've had a couple of people tell me they picked up my book because they respected the person who endorsed it--kind of like an abstract word-of-mouth recommendation.
But what happens when someone you were hoping for says, "Sorry, but no." Make sure you have a back-up plan. And don't wait until the very last minute, or day, or week. I started sending out emails several weeks ago. A good eight weeks in advance of the deadline. One reply came back "no"--the person didn't endorse anything. Another came back "no" because of the religious nature of my book. A third went missing in cyberspace. Two were immediate "yes". And once I figured out that the awol email hadn't gone through, I was able to redirect it to the right email address. So I'll now have three great endorsements.
Let's turn the tables for a moment. What if someone asks YOU for an endorsement? What are the guidelines for reading an ARC or manuscript, then deciding if you want to endorse the work? Do you read the entire thing? A few chapters? Well, you need to decide in advance because it's your name on the endorsement.
I was asked once to write an endorsement and I had to turn it down. The press deadline was a week away. But the author asked me the day before I was scheduled for inducement to deliver my baby. I knew I couldn't do the book justice and write a fair endorsement. Especially since I still had last minute things to get ready for the big day.
But overall, I like to read the entire book in order to write an endorsement. I haven't come across a situation yet that I've had to flatly turn someone down because I haven't enjoyed their work. If you do decide to endorse someone else's book, be sure to read other endorsements to get the feel of them. They are usually 1-3 sentences long. And don't be surprised if the final version is cut and edited from what you originally submitted.