If you want a book to learn or better understand Adobe Photoshop Lightroom, I can recommend Mikkel Aaland's Photoshop Lightroom Adventure (O'Reilly Media) as a good option.
But you'd better move quickly. Lightroom is changing fast--probably a lot faster than the book publishing business can. The software has moved through beta to 1.0, 1.1, 1.2, and 1.3 in recent months, and at this rate, a major update might not be far off.
I've been a big fan of Lightroom as a way to make shooting "raw" photos--the unprocessed data from higher-end digital cameras--a process that not only isn't so burdensome but also can be downright pleasurable. The software enables some of the fiddling that some liked to do in the darkroom, only it's faster, more flexible, and doesn't involve sniffing nasty chemicals.
But there's no question that Lightroom comes with a learning curve, even if you're familiar with the ordinary Photoshop software. Not only do some new tools and adjustments appear, but the task-oriented interface is a dramatic departure for Adobe.
There are oodles of online training resources for Lightroom, from Adobe and others. Aaland's book generally holds its own here, walking users through all the tools and modules. It's amply illustrated with luscious photos he and colleagues took on a trip to Iceland, photos that are useful both to compare the effects of various changes and to inspire photographers to go take their own pictures.
I'm reasonably well versed with Lightroom, but I found several good tips. I liked the background on how the default sharpening setting of "25" is not some arbitrary number but in fact a point on a scale adjusted for each camera model's raw-image characteristics. And he convinced me that the split toning control might indeed be worth messing with on some occasions.
For me, what set the book apart from online tutorials is its collection of recipes--the formulas that photographers featured in the book employ to achieve various effects. A couple I didn't care for (the edited geyser picture on pages 190-191 was tinted a shade of green that matched my own after I saw it), but some others are interesting or handy. One is the Bergmanesque effect on page 220 that's in vogue in fashion photography these days. And who knew all the wacky things that can be done with the camera calibration settings?
One of the things I like best about Lightroom is that it's suited for my style of photography, which tends to be mostly grounded in reality. Unlike regular Photoshop, Lightroom isn't engineered for Impressionist-painter special effects, 3D chromelike text overlays, and multilayer compositing. But images do benefit from some adjustment and refinement, and the recipes give good suggestions and examples of what can be done.
In short, Aaland's book is a good guide for Lightroom newcomers and beyond.