Hi Darkyd10 and welcome to the mtt strategy board!
I hope you will go on with posting your questions and ideas in the future. I would be happy to assist
Regarding your questions:
I would recommend searching for a skype group where other mtt players try to improve their game by doing session reviews together, sweating each others sessions and talking about hands and strategy. Such a group is not easy to find, but if you do, you will see big improvements for sure.
In general it's important to get input from other players since you won't realize all of your leaks by yourself.
I would also recommend watching a lot of videos, posting hands in our hand evaluation forums and post your questions in the strategy forums.
Books are of less importance imo. There are no "must reads" and most of the books are outdated and at the same time overpriced.
Furthermore I can't agree with Alverine.
ICM is one of the most important concepts in late game and final table play. Since this is the spot where you win or lose the most money you should obviously improve this part of your game as well. To get deep you should learn chip EV push/fold/call decisions too, of course. But you shouldn't erase ICM from your list - you should at least know the basics and practise some of them in the ICM Trainer.
Betsizing in MTT's is quite easy. Regulars will start with 3 to 5BB in the early stages and reduce their open raising size down to 2BB when antes hit.
Postflop the betsizing very much depends on your opponents, the board structure (dry board or lot of draws?) and of course your own hand as well as the preflop action. In most cases 40 to 60% potsize (depending on the factors above) is absolutely sufficient.
Last but not least the late stage:
Try to be active. Steal from late position, start restealing a lot with 15 to 25BB, try to abuse money bubbles by playing even more hands (being a bigstack).
You may have a look at all of our mtt videos where whole tournaments are reviewed. The last one or two videos of a series are covering late game play in most cases. Some of them cover short stack play for sure.
Hope I was able to help.
If you have any further questions feel free to ask!
Regards,
Asaban