Sites
https://www.diymobileaudio.com/ - a car audio forum focused on advanced system design. While the site mainly serves people interested in Sound Quality over SPL, it’s a great resource for system building a group of responsive people who will help anyone with car audio questions.
https://www.the12volt.com/caraudio/caraudio.asp - a site dedicated to beginner car audio. There are sub box calculators, diagrams, and information on everything from equipment selection to the perception of audio.
Audiofrog Straightforward Guide to Tuning - an introductory guide to tuning a system for SQ using a DSP.
Books
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms
Floyd Toole
A comprehensive discussion of audio and acoustics. Toole breaks down the physics of acoustics as well as how the brain interprets sound. A great read for understanding how audio works.
F. Alton Everest and Ken C. Pohlmann
Explains the art and science of room acoustics and architecture by combining theoretical instruction with matter-of-fact engineering advice. A great book for understanding how a room (or vehicle interior) impacts sound and how to design around your environment.