Okay, so I have a few things to say, and I don't want to come out rude, I want to help.
1)Breaking a horse is an art: of body language and grounded energy of the trainer, this means they need experience and the fact that you are looking for books sounds to be like you don't really have experience. That is not a bad thing, it just means you won't be able to properly train this horse to a medium standard. For your safety and the potential physical and/or mental safety of the horse I advise you not to continue with this or hire a trainer to help you. If you do hire a trainer and make sure you learn as much as possible for your money!
If you haven't ridden a horse this young, with the guidance of the instructor, this makes me even more worried.
2) I highly disagree with starting horses at 2. At 2 their bones have not fused and their back is most definitely not strong enough to hold a rider, although this is a fair sized horse and you are a lightweight rider, you may be able to do very
light riding but to do that you must be a very balanced rider.
At 2 you may start with light work and start getting the horse to gain muscle before you start work. Only large lunge circles should be done at the trot and canter, but any size at the walk.
Also, at 2 a horse is not mentally ready and without doing anything on purpose the horse may cause harm to you.
3) Being an OTTB, this will probably be even worse, because you will have to undo all the race training this young horse has had.
4)You are leasing this horse, so you are paying for the costs of the board and the lease. If you train this horse, on your tab the horse's value will increase and the owner can sell the horse for more. I disagree with this and believe you may be being taken advantage of but I don't know the details so I may be wrong.
Finally, through personal experience I advise you not to do this! I did the same thing...
. It's fun, exiting and challenging training a young horse and I really wanted to do it. I bought a young horse and had fun, I thought I was training it really well and then 2 years later I moved to a proper stable and they said "your horse isn't even broke to ride properly". The horse accepted me on its back but it wasn't really trained because I didn't know how to train a horse, but I thought I could. I ended up paying $10,000 to get this horse trained and now its an amazing horse.
Training a horse requires many many things but the most important thing is many years of experience. You need to have had lessons with many different instructors, have had experience with all sorts of easy and difficult horses of all ages and sizes before you think of going at a horse by yourself. You will get to a point where you don't know what you did wrong, and what you did wrong was not knowing.
To solidly train a horse you must be a grounded person who is able to show dominance to any horse without abusing it mentally or physically. You must know your limits in praising the horse and punishing the horse. You must have control over all you body actions and balance because you have no idea how in tune horses are with body actions. You must be able to stay confident through any difficulties. And most of all, you must recognize that anything the horse does wrong is not the horse's fault, but your fault in somehow being ambiguous or ignorant. You may never anthropomorphisize the horse, and you must know that horses are always innocent
If you are ever second guessing yourself then you just aren't ready. I know this wasn't what your thread was asking, and this post will probably be deleted, this is what I think for your sake and your horse's sake.
As a warning, do not get sucked into a bunch of Natural Horsemanship BS because most of that is a scam and never gets anywhere. To do real Natural Horsemanship you have to learn from a real horseman. Natural Horsemanship also often leads to a person being submissive with a horse, and being too nice to a horse. So, only do it if you really know what you are doing. Using bits, whips and spurs isn't bad but misusing them is. A whip should only ever touch the horse as tapping or in an extremely dangerous situation(which shouldn't occur in the first place if you are experienced enough). Without enough experience(and you will know when you are ready), only a snaffle should be used.
I really hope I've helped and I hope you think about everything that I've saide.