I'm in a similar position, I want to learn both eventually but chose to start with Rust because it has several really strong-seeming (like the intro book, or Rust Atomics and Locks), while Zig doesn't have many books yet.

The official Zig language reference covers everything you need to know about the language to use it: https://ziglang.org/documentation/master

It links out to the standard library documentation, which is also comprehensive and easy to navigate: https://ziglang.org/documentation/master/std

The need for book-length expositions is much greater for Rust.