You can buy a book and there's certainly a lot of good books out there but you can learn as much, if not more, but just following Adobe's tutorials as well as the hundreds of blogs out there describing different techniques. There are a few websites you might want to bookmark. First is
Lightroom forum it's an excellent source of information, especially if you're stuck on a single issue. It is run by Victoria Bampton, author of several Lightroom books. Second is Julieanne Kost, a self described "Adobe evangelist". She is extremely knowledgeable and her videos are second to none. You can find here Lightroom 4 videos
HERE. And finally there's Adobe's Lightroom site that has links to a great many tutorials -
HERE There's a lot to learn with a program as heavy as Lightroom so take your time and don't rush things. Once you understand the work-flow design it becomes much easier. This is especially true for PhotoShop. Not so intuitive, IMO, but once you get a feel for how they do things, it starts to make sense.
Enjoy!
Dave