While there are many tutorials and references online, I love having the books so I can look at them when my eyes don't like the computer screen. Chained is still one of my go-tos. I also on occasion like to pack them to take to conventions just in case someone wants a visual example of a pattern that I might not have on my table.
The first book I bought a few years ago. It was something like 'chainmail jewelry'. Learned it with that book and loved it. Unfortunately I lost it, I have no idea where it is and have looked everywhere :-(. I have chained and chainmaille workshop as E-books, both are great. And recently I got from my boyfriend 'advanced chainmaille jewelry workshop'. Can't wait to start on the scale necklaces. Love them!
While most can be learned online, I do like some of the tutorials that can only be found in the books or special projects that are located in there. The Chained book is one where you can either buy tuts separately or buy the whole book and get all of them covered. They also have some unique views on weaves that can be worth the cost or at least maybe a trip to the library. The Karen Karon books have some great tutorials in there and project ideas.
I like the books for inspiration and details. I love to look at things online, but for me I'm a paper hoarder and end up printing to look at. My one tip for some of the books is to take them to a Staples or similar and have the spines cut off and the books put into spiral bound books. Much easier to lay down on your table and easier to flip through without losing your place.
I have quite a few of the books-some of the instructions aren't useful to me, but I love the ideas. One of my current favorites is Chain Mail + Color. There is a nice write up in there about aluminum which I have found helpful for selling points. I never realized how much history there was behind a metal we take for granted!