I'm a rabid Safari user. I think they built their whole business plan for me personally... okay, that might be a bit arrogant but at least I know I'm not alone in my rampant desire for technical books and my need to constantly keep an updated library for the problems/platforms at hand.
1. They've done a great job segmenting the pages to load in pieces so browsing through books is much faster. There seems to be heave leverage of Ajax involved in that process.
2. Search is much, much better. Autocomplete when you enter search terms, faster response time, and a slider control that lets you sort by popularity or by relevancy. Very, very cool.
3. Expand / Contract left navigation (table of contents, search results)
I'm sure there are other improvements but getting search and printing to work better for a site offering online books is key.
Just put Ruby in a Nutshell on my bookshelf, preparation for RubyCLR.