I don't have a book I'd suggest adding to the reading list about Hekate that isn't already on there. But I do have a recommendation that's one of my favourite books about magick.
'Spell Bound: A new witch's guide to crafting the future' by Chaweon Koo. It's about glamour magick. And it's syncretic. But it's way more fun than 99% of beginner-friendly books on magick. It also contains original thought. Which is rarer than it should be!
Not "the best" or most "authoritative" or "important" like all the stuffy books you're "supposed" to read. But a genuine heartfelt personal favourite that I think the community might enjoy. I properly loved reading this book. And I legit think it's one of the most interesting magickal books I've read.
Also: The illustrations are gorgeous. I know you're supposed to read books and not just look at the pictures. But once you see the book you'll understand what I mean. It's a magickal talisman in book form.
Here's a picture of Hekate from the book. That seems fitting.
Hope that helps!