Booklets are an interesting area to collect. There are only about 40 different ones issued. Even if you include all the minor varieties (different ads on the back, plate errors such as the "IsraeI" variety, etc) that only brings the total up to about 55. It works out to less then one a year. The majority of the booklets are also inexpensive, with the exceptions being the first 6. The first 6 (plus there varieties) range from around $60 to over $600. All the ones after that are in the $5 to $20 range.
It is also possible to find some minor differences in the booklets that make hunting through a box of stamps more fun. For example, this months of the year self adhesive stamp booklet was originally issued in 2002, but was reprinted in 2003. Since booklets are not listed in the Scott catalog, most dealers don't realize the second printing occurred. By looking at the date on the back of the booklet (the one on the left is from 2002), you can perhaps find one that is missing from your collection.