As a micro/low stakes player who's been around since the Moneymaker poker boom yes all the old Harington, 2+2 books and most good/great books from that era are still fine and a good base to build off and still be a winning player. FWIW I would recommend Peter Clarke's "From the Ground Up" course at runitonce poker. You can't beat the price $50usd. You wanna get a sample of his coaching style first search him on youtube "carrot poker/corner".
With that all said I only play at
pokerstars and play full-ring tables. In my opinion it's a pretty soft/easier player pool because most the better or good micro regs play 6 max tables or faster games because they provide much better volume. I'm far from a decent cash player as I've been playing mainly MTT's for 15 years. Other then 1 bad day since Oct I'm crushing 2nl for 12+bb/100. I just don't get much volume because lack of time and playing full-ring tables. I also only double table(but not all the time). I fully admit my cash game is rusty and I'm still make some pretty terrible and costly mistakes.
The only real difference from the Moneymaker poker boom till now is just less fish by alot. The fish out there still suck and are terrible as they ever were though. At least at Pokerstars full-ring micro stakes(2nl/5nl).
I study and review my hands weekly. I also replay over and over again Peter's course videos while studying/playing. Which now I do less because well I know the content so well now. I also use PT4 "leak-tracker" so I know what parts of my game to work on and spots I need to study.
Cheers!!!