For DoomRL, passive defense traits like Iro and TaN are not going to be as useful as things that help with offense (SoG, SoB, Bru and the like) or things that help with
active defense (HR, DM, Int and friends). The reason is that things hit really hard in the game, while you don't hit as hard unless you increase your offensive capabilities. Iro and TaN can increase your survivability against an opponent by at most 2 more hits than without, but you need to kill them fast enough to take advantage of the extended survivability as the enemies get harder and harder.
Thomas' old guide is a bit dated but introduces the concepts we talk about on the forums, while
Sylph's addendum adds on to that. It's also dated, but not as bad. Finally there's
dtsund's beginner's guide that tries to reduce the other guides into a pithy form that covers most of the gameplay from a beginner's viewpoint.