Adept1 is 1%/sec, and 6%/sec healed while berserk.
Adept3 is 1%/sec, and 7%/sec healed while berserk.
Not a huge difference, but if it ticks and saves your life, the Adept3 will put you a couple of hundred hp better off at the end.
I upgrade everything to Adept3 anyway, but if you get the book and are short on rares, it would probably be fine to leave this for a later upgrade.