The physical quality of the books being sold is generally excellent -- I've bought some that were hard to tell they were used, though a few show a bit of wear. But it's the price that's bizarre. Hardcovers are generally just a dollar. Paperbacks are a mere 25 cents. (I've occasionally found myself wondering, "Do I really want this paperback?" -- and then remembering that it's only a quarter, so I grab it.)
The sale runs from 9AM to noon. It's a bit of a drive from Los Angeles, but fun to do every once in a while. I don't go every month, particularly since it takes a few months for the selection to see much of a change (and even then, it doesn't change drastically). This barebones website here gives information about the event. Don't be put off by the fact that they haven't changed the details since 2011.
The website isn't incredibly clear. The Hermosa Beach library proper is on Pier Avenue. The sale is at a small facility around the corner on Bard Street, across from the fire deparment.
The library itself has a nice, ongoing sale, as well. It's only paperbacks, but all are in near-mint condition. I forget the prices there, but I think they range between 25 cents and 50 cents. The library opens at 10 AM.