Originally Posted by JD Gumby
It means the book equivalent of movie summer "blockbusters" - mostly big, shallow, easy to read stuff that doesn't make you think much, which you can read in short bursts. The type of thing you'd spot mom-types reading in a beach chair while their kids are off in the water.
I also think it plays into a phenomenon similar to marketing certain items for New Years Resolutions. I've known a lot of people, especially females, that spend all year claiming they don't have time to read, but "This summer I'm going to..." Sometimes it's because they have relaxing vacations planned, or the kids are being shipped off to the grandparents for a few weeks, etc.