Josef Fritzl, 74, fathered seven children born to Elisabeth Fritzl, 43, during the time he imprisoned her in the cellar of the family home in Amstetten, Austria, Britain's Daily Telegraph reported Monday.
Fritzl was convicted in March 2009 after pleading guilty to multiple charges including rape and the murder of one of the children he fathered.
He has been writing every two weeks since then, asking Elisabeth for money "to study to be a lawyer so I can defend myself," asking for more than $500 to "get started" on a legal career, the Telegraph said.
Fritzl's sister-in-law Christine revealed the details of letters in which he complains, "that I can't even get a Cola" or special shampoo for his hair implants, the Telegraph reported.
Elisabeth is being looked after by residents of the village where she is building a new life, Christine said.
"She likes wearing snazzy jeans and is proud of her children, who all go to school and are doing well," she said.
"She likes doing all the things she could never do in 24 years inside that cellar."