Walter Gibson (1897 - 1985) was a magician and prolific author. He wrote dozens of magic books under his own name, and was ghostwriter for books "written" by other magicians including Howard Thurston, Harry Blackstone Sr. and Houdini.
Gibson was very close to Houdini and Bess. After Houdini's death, he assisted Bess with the annual Houdini seances. When she decided to end her involvement in 1937, Gibson took over the responsibility and faithfully carried on the tradition each year until his own death in 1985.
This envelope was postmarked on October 20, 1926 in Philadelphia. It is addressed to Houdini at the Princess Theatre in Montreal, Canada.
Houdini was performing at this theater when, on the afternoon of October 22, he suffered an unexpected punch to the stomach while reading his mail- perhaps this very letter from Walter Gibson! The punch ruptured an already-infected appendix, causing the peritonitis which killed Houdini nine days later.