The best way to learn about agencies is to work in the office of one. Forget the how to books. Once you are working "in" the industry, meaning in the office not as an agency nurse, you'll have a chance to decide if it's right for you. You may find you don't like it, and that means you just saved yourself a great deal of time and expense. To the contrary, you may find you really enjoy it, and now you've learned more than any book can teach you and are more confident that it's a reasonable venture to undertake. The best mechanics don't always make good shop owners. The best pilots don't necessarily make good airline execs. The best cooks don't always make the best restaurant owners. I think you get the point.
My stance on most of those how to guides and books is that if the people writing them could have really "made it" in the agency business they wouldn't be trying to sell you information packets. Ask yourself, why would a successful person tell you how to potentially compete with them for a few hundred bucks? If it's so easy to get rich owning a nurse agency, why are they bothering selling a lousy book? They should be too busy raking in their millions to waste their time peddling a book.
I like to fish. I love to brag about a great catch and show all the pictures, but I NEVER tell people where I caught them. If I did that, the next time I visit one of my favorite spots I may be competing with a bunch of other fishermen. Granted, they may not know what kind of lure or bait to use, but just the noise and the space they occupy could scare off the fish or make it difficult for ME to fish where I want to. Everyone has the same opportunity to find their own fish, but I'm not going to tell them how and where to catch MINE!
Likewise, a successful business person likes to brag about their accomplishments and enjoys the fruits (money) of their labor, but they aren't going to tell you how to get that for yourself lest you take THEIRS!