BOB stamps (short for Back of the Book) were special stamps issued for any specific purpose other than regular surface mail. These stamps are usually listed in the Unitrade catalogue with a specific letter preceding a number (for example: Canadian perforated 5-Hole OHMS items start with "OA"). This grouping includes Canadian stamps such Semi-Officials, Officials, War Tax, Registration, Special Delivery & Air Mail. 

This area of collecting has grown rapidly in popularity over the recent years as many groups still have a lot room for research & discoveries!