I was never much of a poker literature guy so can't recommend any specific books. Of course guys like Brunson, Sklansky and Harrington are absolute beasts (especially Sklansky imo), but compared to modern standards, they would get beaten by the high stakes regs that plays today online (in a live setting they would still hold their own easily because their knowledge is deep and experience is unmatched).
Modern Poker Theory seems like a very good book, I haven't read but looks helpful. Anything that can teach you the concepts of GTO is going to benifit you if you can learn to understand and implement it to your game.
Even watching some breakdowns from Doug Polk like he used to do on his Youtube channel can be very helpful, Doug is probably the most GTO-player when it comes to heads-up poker, and just hearing him analyzing hands is both entertaining and educational.
Mid-stakes cash game or mid-stakes MTTs by the way? Private coaching is better for cash game and courses like RYE better for MTTs (bencb is a true tournament beast). For tournament I say the most important thing is ICM, because it changes up how the game should be played. If you are a shortstack, it doesn't change but with a middle stack, you want to bluff a lot less, and hope to climb the prize ladder without losing chips. When you're shortstack it plays different and when you're chipleading you are supposed to put pressure on the others.
Back when you played poker it was probably standard to raise 3x-4x BB in both cashgame and MTTs and nowadays, the preflop sizes and also the continutation bets are a lot smaller. Look twitch-streamers who play poker, guys like innerpsycho for cashgame clues (he speaks russian/english but you can still see how he plays, he's pretty ABC player but very solid, winning NL500 reg), guys like bencb (Raiseyouredge on twitch) bigbluffzinc for MTTs, both very solid highstakes MTT regs.
I wouldn't recommend coaching until you are a winning player but want to win even more. If you are losing player coaching can't really help. Sure it can fix a few mistakes but most likely you make a lot more. Best tool for me to get back into the grind was just watching the top-regs and copying their playstyle. Pay attention, and grind a tourney/cash game table on the side and try different concepts as you go along. Listen to what they say when they explain their thought processes, and don't be afraid to ask if you think they play weird, because with 99% certainty it's not weird, but they have valid reasons for what they do. Sometimes it can be small details like "blockers", this is a concept from Omaha, but the old guard like Brunson & co never really knew about these details, that's why I go for something more modern books over the old ones. Also back in the days they didn't really float flops a lot with backdoor draws, but since times have changed and betsizes got smaller, it's very often worth considering even backdoor draws, 4% chance for a backdoor flushdraw doesn't sound like much but when you a millions of hands, we're talking about 40 000 flushes.
Buy that modern theory book (and let me know if I should buy it too)), a poker HUD like Pokertracker or Holdem Manager (this is for getting fast info on how often people enters pots, bets, raises etc), and then save your money and instead use the free tools accessible, like Equilab for understanding how different ranges plays against each other. Update your knowledge around pot odds, realizing equity and listen to the pros who give all this free info while streaming.
It's a lot of work, but you just do it little by little and you will get better and better. Use the PT or HEM software to save any interesting hands you might not be sure if you played correctly or not, and you can analyze it later with Equilab or later if you decide to speak to a coach.
And for cashgame the absolute most important thing is table selection. Unless you play zoom/fastforward you need to be very careful who you decide to play. Back in the days you probably (like me) just joined any random table and sat there until no one was left or you got bored and left. Nowadays the regs basically are hunting fish. As soon as a the fish is busted from the table, the other 5 regs on the table just leaves. It's simply not worth the time or effort to play against equally strong players, the money comes from spotting and exploiting the fishes. If you make a comeback be ready you will be hunted by regs every time you sit down until they either realize you good enough to hang with them, or until you are busted. Definitely wouldn't recommend just jumping on midstakes cashgame, it's way harder than it was 15-20 years ago.