Lindsay Lohan -- The Secret Jailhouse Hobby

Lindsay Lohan used her time behind bars to brush up on one of her favorite hobbies ... writing -- and TMZ has learned she plans on turning her work into a profit. 



Sources connected to the jailbird tell us Lohan filled up several notebooks during her incarceration ... documenting "every single thing about her life" in jail ... in the hopes that she can eventually turn the "journal" into a book.

We're told Lohan wrote about everything from her relationships to her family -- and even scribbled about her dreams.

It's not the first time Lindsay's tried to get her writing published ... just ask Judge Revel.