You are in an excellent position to start preparing for the UMAT, but still consider what everyone here is saying as well. Year 10 is a good chill year, don't go overboard on any books, questions or practice courses but you can still do some small things to help you with the UMAT.
Start being absolutely anal about analysing anything at all, from ads on TV to fine print on posters to whatever you read in the news - try to understant what a reasonable conclusion is and how not to make assumptions. Start being scrutinous and skeptical - it helps a lot for the UMAT.
Try taking up chess or some other activity to encourage mental stimulation. Maybe play some sudoku while on the trains, or even better, DON'T give up on PC gaming! Seriously, take up on a good immersive RPG or best of all a strategic RTS. I honestly reckon that (sparing FPS) that many video games can help sharpen your cognitive skills. Odds are, you are good at chess, solving a sudoku puzzle and skyrim - then you are probably going to be better at S3 than the average applicant - (especially chess, seriously get on that).
Otherwise, if you want to, there are certain subjects that can also help with the UMAT - namely biology. I find that it helps me a lot with section 1, especially considering there are a lot of questions in S1 that test your understanding of scientific method, which is best taught in bio imo. Not to mention how often the contexts of the S1 questions are related to biology.
This is a personal preference (maybe) - but I discourage speed reading actively, especially to people who I teach the UMAT to who try and use it to poor effect in S1. I am not saying it is bad inherently, but the way people practice it makes it:
1. not very useful in the UMAT: the amount of time you spend reading in the UMAT = very very little, increasing your speed won't help much,
2. possibly harmful - if by speed reading, you mean skimming through the question and not taking your time to process everything slowly then don't try it. Everytime I see someone try this, they fail to read something important.
If you do want to read anything, maybe try some sophisticated novels etc - learning new words and expanding your vocab is the best way to prepare for S2.
Furthermore, if you do want to be a bit hardcore then you could also consider getting some books to practice some logical reasoning questions (you can also find some generic ones on the internet too). Seriously, anything you can get your hands on, brain teasers, little problems whatever, they all help.
Also... It may not be too bad to even consider getting a course at this stage. Sure it is a ridiculous amount of money to spend, especially when you are 2 years away from the UMAT, but maybe you could in fact look into something like Section Zero, because it is much cheaper (probably costs about as much as 3 of those problem solving books you could buy) and you could have access to the course for all 3 years if you ask. (normally it is 2, but see how you go).
All in all, don't go too overboard at this stage, just partake in some activities that you find fun that can still help you with the UMAT (even if that does mean revisiting AOE every now and then)