To the OP, actually, the best way to prepare for technical diving is by diving a lot and refining your skills, and by increasing your experience (if your number of dives is up to date - that's rather low). The best way to do it is by diving together with some other tech divers if you can. Also, try to speak with some instructors.
However, I understand you are willing to read and improve your knowledge; it's the same for me
I can tell you that you can do (at least) three different kinds of things:
1) work on your physical conditioning
2) read books
3) work on the equipment
1) work on your physical conditioning
People tend to agree that tech divers should have proper physical conditioning. You should work in two directions:
{A.1} Increasing your strength: you don't need to be a powerlifter, but tech equipment can be rather heavy (easily 40kg+ - up to 60/70 or even more, if you add stages, scooters, camera, etc.). No, you don't need to squat 100kg
But some squat/deadlift and benches and chin/pull-ups may help.
{B.1} Increasing your aerobic conditioning: this is important for several reasons; again, no need to be an athlete, but being able to run at least 8km in less than one hour is a good starting point. Alternatively, go swimming or biking.
2) read books
Here there are a couple of things you can do; in order, I would:
{A.2} focus on the diving philosophy: you may know that different agencies advocate different styles of diving. There is no best way of diving, but the best for you exists. You may want to read something related to GUE (the book from Jablonski previously mentioned), something about TDI (books from Mark Powel) etc.
{B.2} focus on physiology: there is an excellent book from Mark Powell, deco for divers, easy to read and very explicative (although, as a doctor, I found mistakes in tables and graphs). I also have a book from Bennet and Elliot, but it is more for physicians, I believe (800+ pages!), and I open it only when I have some particular doubts (and there is another one in the market, but I can't remember the title)
{C.2} focus on the psychological aspects - that is, human factors: here you can read the book from Gareth Lock (some people say that this book says nothing new and overcomplicate everything - I disagree, I find it interesting). You can also try to read books describing accidents.
{D.2} focus on the equipment: some books provide a general overview of some specific pieces of equipment and how they work (I am specifically thinking about rebreather here, but probably there are books about other stuff); I believe books from {A.2} already cover the basics you need to know.
3) work on the equipment
Again, different things that you can do, but these are a bit risky (I would personally skip them):
{A.3} Exercises: assuming you finally choose a style of diving and the equipment you want to use, there are some easy exercises that you may emulate. For instance, if you choose to dive with back-mounted manifold twink tanks, you can simulate a valve drill using only water plastic bottles (really!). But this can be very risky. The principle of these exercises is to train your muscle memory. So, if you do them wrong, you'll learn something terrible that you will later need to forget (and the process of forgetting will be tough)
{B.3} getting proficient with the equipment: if you already have the equipment that you will use, you can spend a bit of time playing with it to understand how it really works and become a bit more proficient. But you first need to buy the equipment, which can be expensive and, in my opinion, is never a good idea (I believe that the best idea is to rent the equipment together with an instructor, test it, and buy only if you like it)
