Answer

I don’t have any recommendations. Personally, I hardly deal with them. But each person should go with what speaks to him. I also don’t accept the distinction implicit in your words between Jewish study books and those that are not. Books of thought by Maimonides or the Kuzari are no different from Kant’s books. They are all books of thought, the product of the author’s own thinking (and not Torah from Sinai), and as such each person chooses what speaks to him and builds himself through it.
In the book I am currently working on about contemporary theology, I expand on this point. See about this in two articles on my site:
The Natural Sciences and the Torah
The Boundaries Between Sacred and Secular Literature in the Library