I've been been an off-and-on yoga practitioner since my twenties. I started off with simple routines that were not all that strenuous. I suppose you could say they were designed more for stretching and relaxation rather than building muscle strength. Certainly I'm no expert, but here are some yoga books I like, for those of you who may have an interest in trying it:
Yoga Self-Taught — Andre Van Lysbeth
Richard Hittleman's 28 Day Exercise Plan — Richard Hittleman
Journey Into Power — Baron Baptiste
40 Days to Personal Revolution — Baron Baptiste
Probably the most accessible one of this bunch is Richard Hittleman's book. There's not a lot of text compared to the others. Generally he just has one page explaining the basis of that day's routine, and the rest of the pages for that day show photos of the poses and detailed explanations on how to perform them. Good for anyone looking for a relaxing, not-terribly-time-consuming routine.
The other accessible one is the Yoga Self-Taught. This one does take quite a bit more reading, and espouses a couple of odd diet things, but the poses in here are classic poses—the very basics you need to practice yoga.
Baptiste is somewhat controversial in yoga, from what I understand, but I really like his routines. His books could stand another once-over in the proofreading department, but if you don't like learning from a book, you can always pick up one of his DVDs. (I'm not sure yet what I think about the "personal revolution" part of the 40 Days book, or if I believe you can "Journey into Power" But the routines in both are good.) Although you should be warned that the style of yoga he teaches, vinyasa yoga (aka "power" yoga) is intense and the routines are long. Still, his routines get results. I've been using 40 Days for the last two months (taking twice as long to move on with the routines than recommended in the book), and so far, so good. My arms are more toned, my stomach is smaller, and in general I'm stronger than I was.
There are tons of other books, DVDs, and so forth from which to choose, of course, but I thought I'd throw these up there.