Caroline,
In Eaton and Haas' book "Titanic: Triumph and Tragedy", there are several pages of reproduced insurance claims made by several passengers of lost items, such as jewelry, money, pieces of intrinsic value, etc. Also lost with the ship were unknown amounts of money, cash, and the Rubiyat jewel-studded book. Unless the money and book were stored in something to protect it from the elements, these are likely gone.
Major Peuchen was said to have left a large amount of stocks in his cabin, but again, paper items not protected in leather bags or in somewhat sealed conditions are likely long ago consumed by organisms.
James Cameron and crew went back to the purser's office in 2005 and found several pieces of rotting furniture (desks, cabinets) as well as a large metal two-handled locker. They did not attempt to open it as far as I know.
In their 2001 expedition, and included in the Ghosts of the Abyss DVD, Cameron's team went into the cargo holds and saw what was interpreted to be a touring car's headlamp, a fender and a tire protruding from the debris under the fender. However, when searching for something like the Renault, they might have been seeing what they WANTED to see. I personally interpreted the debris as they did, but they drew no conclusion as to what the pile of debris actually was, and moved on.