Kit's books are good for anyone wanting to make some parts, customize things, etc. For the person buying all their parts and assembling guns, a lot of what he has in his books is not needed. Making springs for example. Kit was making guns when parts were not available. He was a bit of a shade tree mechanic.
Much has changed since the 1970's and early 1980s when Kit and Chuck Dixon and Wm. Buchele wrote their books. Precarves and kits have advanced and many builders don't need all the info on how to make thimbles, nosecaps, sights, triggers, patchboxes, inlet a barrel, make a ramrod groove, drill the hole, etc. Now builders of kits need to know how to deal with problems specific to kit building (inlet the lock first then move the barrel as needed, because the lock can't be moved). So what was in the old books has to be sifted and maybe modified if you're not building from a blank.