Here's my recommendation for general knowledge about sewing trousers:
A great reference book for trousers is David Page Coffin's "Making Trousers for Men and Women" available in book form and Kindle here. He was an editor for Threads Magazine for many years. You will learn techniques for making high-quality trousers, with nice details like welt pockets, fly fronts, belt loops, etc. I'm not affiliated with him or Threads magazine.