This a pdf (36 pages) of the trouser making section from Cabrera's tailoring book - the 1983 menswear edition. Cabrera's book was considered as something of a standard text for the Basic Apprentice's forum. It is a very clear text with good illustrations.
Anyone starting from scratch can and probably should begin with this.
Cabrera: Trousers (https://www.dropbox.com/s/0b64700hemq3slm/Classic%20Tailoring%20Techniques%20-%20Trousers-R.Cabrera%20%281983%29.pdf?dl=0) (3mb)