I ride Books B-17 on all 3 of my touring bikes. The oldest has over 12,000 miles on it. People either love or hate them. I'm the former. My wife rode the women's version for over 600 miles and suffered every minute she was on it. She sent it back to Rivendell mid-tour.

It takes maybe 1000 miles to fully break in a B-17, though my 2 newer ones cost extra because they were partially broken in and got pretty comfortable in maybe 1/3 as many miles. You certainly don't want to take a brand new one on an extended tour, especially if you aren't sure it's right for you.

You don't really need springs. It's high quality tensioned leather. "Breaking in" means the weight of your body sitting on it shapes the leather to your sit bones, like repeatedly catching a baseball shapes a glove to your hand. After 100 miles or so, the saddle fits your butt like a glove, giving where it should and supporting firmly where it needs to. The design hasn't changed in a century because it works. But as I said, some people love them, some hate them. Find out which group you are in before taking one on an extended tour.