Ok, so I have some stuff to unpack:
First, I am coming back from roughly a 6 or 7 year hiatus from playing. My approach to playing was based more around guys like Ivey, Dwan, Galfond, etc. during 2009.
I would say that back then, I was definitely more of an advanced player. But I wasn’t the kind of guy that could crush a table of really good $2/4 6-max Regs.
So, I’m looking to update my approach. One thing that has really changed since I was playing is bet sizing; i.e things like the “downbet”. When I was playing, and there was a 3-Bet pot, the player would bet between 55% - 80% pot on the flop. The smaller bets would be used on the dry boards e.g K-7-4 rainbow. And the bigger bets would be used for wetter boards.
But now, I see a ton of 25% - 40% bets in 3 & 4 bet pots.
Another thing that has changed is that I often hear things like “you should always be calling when you’re at the top of your range”.
These concepts are all new to me, and I’m looking to further my understanding of the concepts.
If anyone could recommend some books or forums that can help me understand these new aspects of the game, I would greatly appreciate it.
Sorry if this is a little vague but I’m relatively new to the forums.

Are you playing live or online? (Book recommendations differ according to the game you're playing).
For online, The Grinder's Manual is a decent primer with some info on bet-sizing in various spots. (Although even that is a little outdated now. It's good for newcomers and returners to the game though).

I will be playing live. I would play online but I’m pretty sure there’s no site that allow players from Illinois to play.
Ya, I’ve heard about the grinders manual.
Are there any that are better suited for live play.
Thanks for the reply.