The book "Silent Coup", by Len Colodny and Robert Gettlin, details the story and Liddy has said that they got closer to the truth than anyone.
According to Silent Coup (Book 2 - Golden Boy), Dean's wife Mo was associated with a madam who provided prostitutes to the DNC, for visiting big shots. The burglery was an attempt to get the "little black book", which was in the desk of a secretary, Ida Wells (who unsuccessfully sued Liddy). The burglers had the key to her desk.
This little black book was supposed to contain the names of the prostitutes and the Johns. The burglers were also to tap Ms. Wells' phone, since she was the one who made the "contacts".