Two quick questions.

1) In the character's record screen, THACO is listed as xx(xx). What is the difference between the two numbers? Is the first the innate one (from stats & level) and the second after equipment bonuses are factored in? That sort of seems to be the case, but for some of my chars this doesn't work out numerically, at least as far as I can tell, and can't find anything in the manual on this.

2) During battle one frequently sees lines of the type "FOO takes xx points damage from BAR (yy points resisted)". What sort of resistance is it talking about? Is this due to monster bonuses, such as say -2 against slashing weapons?

1) Thac0 - as you surmised, the first number is "natural" thac0 - from class and level. The second number is thac0 modified by strength, racial bonuses, equipment, and magic. High dexterity will modify thac0 if you are using missile weapons - +1 at 16, +2 at 17-18, +3 at 19.

2) That resistance is innate or (rarely) item-related resistance to your attack form. If you look on the record screen and scroll down, you will see a list of the possible resistances. I've noticed that shadows have a little resistance to missile weapons (maybe 10%) as do verbeeg (possibly 25%) and skeletons are 50% resistant to slashing, piercing and missile attacks.

Oops, forgot. For #1, Thac0 is also modified for weapon specialization.