This is the best explanation on the differences between the books. The books have some overlap.
Many years ago, while in college, I read articles by Doc Love, but never fully read "The System". I am familiar with the basic tenets of "The System". I have read "The Rational Male" and am currently reading one of the follow up books, "The Rational Male: The Players Handbook".
The best things from "The System" are the inner game principles of confidence and being a challenge. Most red and black pill guys would agree with the principle of being a challenge. The more modern simping behavior is not being a challenge. Women are sexually repulsed by simps but like simps for getting money on OnlyFans or Instagram. Old school simping, which isn't as bad as new school tech simping, is going to the strip club and not getting laid with a stripper at the strip club.
One of the biggest issues now with "The System" is the changes in technology since it was written in the 1990s. He was writing when home phones were most dominant. Most people didn't even have basic cell phones then, let alone smartphones were text messaging was the primary communication medium.
@EyeBRollin has written about how someone can adapt the Doc Love principles with newer tech forms.
I remember reading Doc Love articles during my college days (2001-2005) and learning from it, but Doc Love work alone doesn't make you red pill. In college, even after reading Doc Love and other PUAs of that era like David DeAngelo, I would say that I became more game aware and might have been a 'Greater Beta'. It wasn't until circa 2010 when I discovered Roosh and other Manosphere writers that I fully took the red pill.
I didn't read "The Game" (2005) and "Mystery Method" (2007) until after I discovered Roosh and others. "Day Bang" (2011) was probably the best pickup manual I read, and "Bang" (2007) was the most useful night game manual I ever read. I did later read "Mystery Method".
"The Rational Male" is one of the best books for developing an overall red pill mindset. Most pickup manuals will help you overall with outer game and probably push you from Average Frustrated Chump/beta male to Greater Beta, but they won't do a lot in terms of inner game, which means that eventually you'll run into relationship problems from a lack of frame (see Iron Rule of Tomassi #1).