How to study poker: http://orangepokerblog.com/2018/03/14/how-to-study-poker-strategy/
PioSOLVER (PIO) is a quasi GTO software program that allows you to input a hand (heads up only) and PIO will give you its GTO solution (ie. what % you should bet (and amount), check, fold, etc. There are hundreds of filters that you can use and you can adjust the situations (bet sizings/ranges/etc). Note: Much of this is going to be predicated on the ranges that you input yourself, so make sure that you are objective when analyzing your hand. As a horse of mine says with these programs: “Garbage in, garbage out”. These programs can hurt you in the long run if you give it highly inaccurate ranges.
Holdem Manager and Poker Tracker
Both Holdem Manager and Poker Tracker are essential for any online poker player wanting to track his results and analyze his play. Without it, it is nearly impossible to go into your stats and leaks. Which one is better? It is a personal preference for every player. I would recommend that you do a free trial at each and see which one fits your needs better.
Poker Snowie (http://www.pokersnowie.com)
Poker Snowie is a poker program that allows you to play against a poker bot as well as do many simulations for specific hand situations (on varying board textures). There are many features within Snowie that are very helpful, even a Pre-flop hand chart for new players.
These are all heavily recommended poker books. Generally, poker books are not as applicable to poker strategy because top end poker strategy changes very often (and the metagame is very fluid). With that said, these poker books are more for the basics and will help a player build his fundamentals (maybe up to 200NL online or slightly higher).
The Grinders Manual by Carroters:
Applications of No Limit Holdem by Matthew Janda:
No Limit Holdem for Advanced Players by Matthew Janda:
The Theory of Poker by David Sklansky
Two Plus Two: http://forumserver.twoplustwo.com/
Poker Strategy: https://www.pokerstrategy.com/forum/
Reddit Poker: https://www.reddit.com/r/poker/
Poker Training Sites
Upswing Poker: Catered for newer poker players, Upswing Poker boasts world class video producers in Ryan Fee and Doug Polk. They use modules to convey poker fundamentals and poker from the ground up.
Run It Once: Headlined with some of the best poker professionals in the world, RIO is designed for seasoned professionals and have a hefty price tag ($100 a month). These videos are more advanced than those at Upswing and for those players playing at least SSNL (50NL+).