I read the book a long time ago and just recently both reread it, and am currently reading the other three books in the saga, which I had not read before. As much as I love westerns, for whatever reason, I'd never seen Lonesome Dove and decided to watch it after the second reading of the book. It was indeed a great mini-series.
One interesting thing I noticed is that if I had not read the book before watching it, I'm not sure I would have liked it as much. Not because of the acting or script, but because the mini-series "scene jumps" in ways that, had I not been prepared for the scene from the book, I would have felt like things were disjointed. I think that actually comes from the mini-series trying to stay as close to the book as possible. It's a nearly 1000 page book, so there's only so much you can squeeze into around a 12 hour TV show. Of course it could be that I felt that way because I
had read the book first.
So definitely watch the series, but read the book first.
Oh, and it's available as (pay to rent or own) streaming on Amazon.