The book Q for Mortals is good. It’s available online for free at https://code.kx.com/q4m3/

Also the reference and blog posts are useful: https://code.kx.com/v2/

It’s not as easy to get information as other languages, but the mailing list is super helpful and all the questions I’ve asked have been replied to by paid Kx (the people that make Q) employees:

https://groups.google.com/forum/m/#!forum/personal-kdbplus