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