I am currently using El Capitan.
Every time I uninstalled iBooks so I could manage my books with iTunes, I found I simply couldn't sign in to iTunes any more. That means I click the 'sign in' button but no sign-in window pops up. I can't purchase or re-download any of my music or books.
And reinstalling iBooks fixes it all.
Is there a way I can manage my books with iTunes and can still sign into my account?