The Dark Heart of Blackhall

 is a Fallout 3 quest that is added to the game with the Point Lookout add-on.

Reward
IF you gave the book to Obadiah Blackhall, you'll get his reward of 1000 caps and 100xp for completing the quest.

Alternatively, destroying the book with the Obelisk in the Dunwich Building gives you a positive karma boost and the 100xp.