This is a fairly complicated discussion including a lot of variables, methodolgies (Zone System etc) and also a lot of people doing things without really understanding why

If you are reading Ansel Adams, stick with it, understand it, and try it. Then evaluate the results by making prints of your test negatives and decide what you like and what you don't. Generally with Zone System testing you are likely to end up with an EI slightly lower than the ISO of the film. Somewhere around 64-80 is probably how most people use FP4+. There are a few reasons for this, but that is a complex discussion on its own. Find your own development times based on Ansel's methodology as well. Again, make prints of your test negatives.

The EI will also depend to some extent on the developer. ID-11 is what can be called a general purpose solvent developer. These are developers that give you a good balance of full film speed (by ISO standards), fine grain and good sharpness. Excellent image quality. It's a good way to start. You can then explore from there. But keep in mind there are always tradeoffs. In comparison to ID-11, you almost never get an improvement in one characteristic (for example, higher speed) without a compromise in one or both of the other main characteristics (grain, sharpness). It is also difficult to generalize about the characteristics of developers because it depends what film they are used with.

Keep things simple. And be sceptical of the claims people make regarding developers, films, dilutions etc., especially when no data or experimental details are provided. Generalizations can be problematic, the interactions between films and developers are complex, and myths abound.

Adams's books are excellent, by the way.