The resource that has been instrumental for me to move up from beginner-that-generally-can-work-out-things-with-a-lot-of-random-tries, to someone who often has things right on the first try, and if not, have a clue of what is going on , has been the following book:

Classic Shell Scripting Arnold Robbins, Nelson H.F. Beebe

Recommend it to anyone with a basic understanding and a will to learn more. It also really doubles as a rich information resource. Cannot recommend it enough if books are your thing.