Suggestions on (good) relationship book, pdf or website for males?

I want to learn how to improve my relationship with my girlfriend, but I am lost in the jungle of overpromising, trivial glossy cover relationship litterature. Most of it also seems to be directed towards 40-something females (i am late 20’s male), and i am lacking a down to earth, no-bullshit, not overpromising resource which provides insight and gives advice that works and is not covered in self-helpish marketing fluff.

Any suggestion on good books, websites, PDFs or other resources (except fluther) where i can read helpful and sane advice on how to improve my relationship with my SO and get insights about relationships without the aforementioned problems?

Books with the above problems but that have helped you alot are ok to suggest as well.