I'm looking for a book to look a bit more into defending (especially when up in material but having no initiative).
How are Aagaard's books?

I think you mean endgames. A pawn ahead is in most  cases a endgame situation. Many mistakes are made in the endgame. 

Defending when ahead.  Me thinks herr Steinitz is rolling in his grave.

No I mean in the middlegame. The title of the book is in the topic.

You can have initiative and still be behind in material. For instance when you gambit?

yeah lol, even so my brain rolls in my skull when i watch steinitz games.