I just discovered Apress's Super Index. It's a full text search across their entire catalog.
You don't necessarily get to see the books online, but if you have a bunch of Apress books on the shelf like I do, you know which one to pull down.
It looks like the images of individual pages will be online later, but when I tried it for my searches, the pages I wanted to see weren't online yet.
I have this with O'Reilly's Safari, too, but only because I got a free account for being an author. SuperIndex is just free.