I'd never paid much attention to scribd before, but had a browse at some of the books there today. Despite being an apparently legitimate site it seems to me that much of the content is illegal. For example AOE 3ed is there, but it looks like a pdf conversion of a scan.
Searching for Hobbs' BEOS it say's they're working with the publishers to offer it. Sounds like bull.
There's a lot of Scitech books there, and they are owned by the IET. I can't see the latter allowing anyone to offer their texts for a few $ subscription a month.