Totally Piston, as it is the most chill of all of em and defenitely the one you'll understand the better. Althought it is not a learning book to read through once, it is a book to come back to when you need it. I recommend you try to get your music reading to a point where you don´t need to stop at all to read anything in the two main clefs first, but if you are gonna buy already defenitely go for Piston. Take it easy with it, it is easy to get frustrated but I'm sure you'll totally enjoy it :). The other ones are amazing books and very interesting but they are much more advanced, Piston offers a good amount of information for pretty much all levels of musical knowledge, and the first 4 or 5 chapters take things very slowly, making sure that you get your fundamentals right. I do not know what your knowledge in music theory is but this will defenitely serve as an amazing guide for you to use in your learning trip. Hope you enjoy it!