I have recently hit the FM title, and this year I plan to try and make IM norms and potentially IM title. Last year I struggled around 2300 USCF because I was usually getting killed by lower rated players. It always takes me a long time to warm up, so I usually destroy my tournament in the first few rounds.
What books should I read? What preparation should I do?
Just out of curiosity: Do you know more precisely why you lost against lower rated opponents? Did you lose in the opening, middlegame or endgame? Did you lose because of time management issues? Also, what do you mean by long time to warm up? What changes for you from "not warmed up" to "warmed up"? Is it level of focus, some mental block dissapearing, or something else?
If you answer questions such as this you may find some things you can improve upon, which may be more important than anything you can read in a book. It may not even be directly related to chess, but it could still affect your chess in an unexpected way.
If you're already an FM, I have a feeling that you already have a good idea of areas of your games that are weak, so I would suggest reflecting about that and then add some details about your specific chess strengths and weaknesses. You may, for example, be an opening expert who isn't really as consistent in endgames as is expected of an FM. Then you may want to study some endgame litterature. Or it may be the other way around; nobody here can know that better than you.
A general advice though, is to try to contact an experienced chess coach, and also, if you have the possibility, talk to strong players(FM+) about ideas for improvements. One other thing you should probably do regardless of your specific situation is to write down and analyze your thoughts during a game as soon as possible after a game. Then it might get easier to pinpoint precise answers to questions like the ones I asked in the beginning, which may hold the key for your improvement plan.
Also, if you're physically unfit, but able to exercise, I suggest you build up some basic fitness. This would most definitely affect your chess in a positive way.
Good luck with your IM ambition. I hope you find a way to get there.
Far be it from me to make recommendations to an FM, but in my contact with a top Soviet trainer who has trained many grandmasters, he has expressed the belief that a good trainer can make a huge difference in developing skill and helping the candidate focus their study efforts appropriately. A trainer can assess your personal weaknesses objectively and recommend ways to overcome them.
For players of all levels, including masters, it is often a good idea to study the endgame because usually that part of their skill is the weakest, so you stand to get the most by studying it. In other words, it is low-hanging fruit.
Personally I have benefitted a lot from endgame study, even though I just an average endgame player, just because other players are so bad at it. Once I beat a candidate master who was 2 pawns up against me in a simple position just because his endgame play was so incredibly bad. The one draw I got against a grandmaster was because I began to outplay him in the endgame (where he had the advantage) and he became frightened so he offered me a draw.
I would recommend Nunn's books, especially "Secrets of Rook Endings".
Also, "Secrets of Pawn Endings" by Karsten Muller.
Learn just those two books solid and you will be amazed how you can beat the crap out of anybody in an endgame.
One final comment about the endgame: often in modern swisses with short time limits you have to play the endgame really fast, somethings blitz sudden death. In those situations the guy who knows the endgame ahead of time will CRUSH the other guy. Even with time on the clock it can be really hard to find the right move in an endgame. You have to know it ahead of time. In a blitz situation, people will play just incredibly bad moves one after the other in an endgame.
