I think you can get most books free from online libraries like archive.org
I recommend anything by Lundy Bancroft, starting with Why does he do that?
How he gets into her head by Don Hennessy is good. As is Healing from Hidden Abuse by Shannon Thomas.
There is an online course called the Freedom Programme. I think it’s £12, but I thought it was just a repeat of the book Living with the Dominator, by Pat Craven, which gets a lot of recommendations on here. I personally didn’t find the book/course as helpful as the other books, maybe because the abuse I experienced was more subtle than what she describes. Having said that, I do like how it compares behaviours of abusive and respectful men. Xxxx