You will probably want to obtain a couple of notebooks (to record some of your work) and a wild flower guide book (detailed key) for the region in which you live, in your own language. My favourite wild flower key, costs around €29 new.
Other books will be suggested in each module but are not required. You can see some of these here. With so much information available online, you will be able to find lots of useful additional material, without buying additional texts. That said, most of my apprentices do like to buy or borrow additional texts as they move along the course. I have accumulated a small library of beautiful herb and nature related texts over the years but this is certainly not required! Being able to correctly identify the plants and to actually experience how to work with them, is our primary focus.