What's up with all the Rust books?

I want to learn Rust, and I've been scouring the internet for frequently recommended, good Rust learning resources (docs, videos, books, courses, etc).

One thing I noticed is the large number of online Rust "books".

How did this happen? I've never seen a community where these books (usually relatively good quality, and totally free) were so common. Any gossip, history, initiative, that explain how the books became so widespread, would be interesting to me. If it's all organic, that's good to know, too.

Examples:

• The Little Book of Rust Books (and this links to many many others)
• Google's Compresensive Rust
• Juniper Book
• Cargo Book

I see that the books are created with mdBook.

Background: I am a software developer with web frontend, backend and mobile experience, and have been following Rust for a while now. I would like to spend the next 1-3 years learning Rust, so that I can get a full time job developing interesting stuff with Rust without having to take a pay cut.

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