The I don’t think the books will be obsolete that soon. Ton will be doing a lot of work under the hood, which will enable things like configurable hotkeys and some new interface features, and especially pave the way for more interface improvements in the future, but the 2.50 interface will not be unrecognizable from what it is now. I’d be very surprised if any default hotkeys changed from what you’re used to without a very good reason.
A lot of new features will be added, and the Bmesh work may result in some differences in how you work with meshes, but Bmesh probably won’t be ready for 2.50, unless I’m mistaken. And even then, the workflow will be based on what Blender users are accustomed to. So learning Blender now will always be an advantage when learning new tools.
I would say, right now, don’t bother learning the current particle system. That will be completely overhauled (actually, it already is, and is available at graphicall.org), and there is so much new functionality that you will have to learn that from scratch regardless. As for the material covered in Essential Blender and Introducing Character Animation, that material will remain mostly accurate (with small changes here and there that you can ask about in this forum) for some time. The only thing is that there will be new features that are not covered in those books. But in order to use and appreciate the new features, you’ll need to know the basics covered in those books anyway.
Still, from my own point of view as a documentation writer, it’s definitely an important challenge to try to future-proof my work as much as possible, which is why my next book on physics and simulations will be delayed until the release of 2.50, so that I can be absolutely sure to have it as up-to-date and future-proofed as possible.