In 1986, magician Peter Monticup (owner of MagicTricks.com) purchased Houdini's personal desk, still filled with envelopes from correspondence with fans, famous magicians and other personalities of the day.
Houdini was a notorious "saver", believing that every part of his life was important and should be documented. (He autographed nearly all of the thousands of books in his library, knowing that his signature greatly increased their value.) Correspondence that he believed was important was carefully saved, envelope and all. The correspondence was put into binders and boxes, and was subsequently auctioned off to collectors in the decades after Houdini's death.
The envelopes, however, were hidden in a secret compartment in Houdini's desk. Houdini received letters from a wide range of people. The envelopes were all sliced open with a letter opener, not raggedly ripped open, and neatly stored in stacks to avoid wrinkles. He actually wrote notes on some of the envelopes. Envelopes from fan letters (some addressed to Houdini while he was performing at various theaters), envelopes from fellow magicians (including Walter Gibson, Svengali, Harry Kellar), envelopes from his bank (still containing his bank statements) and envelopes from psychic research groups and magical associations were found. In the years that we have been researching the names on the envelopes, we've found that every single correspondent had some important connection to Houdini's life- that's why he saved these envelopes and kept them carefully stored away.