In short when I got into Poker originally over 10 years ago, I bought myself a copy of "Hold'em Poker" by Sklansky & Malmuth for a bit of theory study off the table.

Having got back into it recently, after a 10+ year break, I have taken to playing Microstakes Sit & Gos on PokerStars (3 final tables, 2 cashes including a win). Where I hadn't played MTT tournaments before, I was wondering is there any recommended theory books (as I've noted the feel, feels - at least to me - very different from a ring game).