Back covers (Canon)
Although the front cover (justifiably) gets the lion's share of attention, the back covers of the Hardy Boys books have evolved over the years, and can be a useful tool for narrowing down the printing date.Back covers of dust-jacket editions
Dust-jacket editions featured a variety of back covers, usually displaying advertisements for other Grosset & Dunlap series (frequently Tom Swift Jr. in later years). I hope to post some scans at a later date.
Back covers of picture-cover editions
After the publisher switched to "picture cover" (pictorial cloth) format in 1962, the back cover was dedicated to a list of volumes in the Hardy Boys series. The list was updated fairly regularly as new volumes were published.
Two major designs were used in the PC era. The first is the "Emblem" cover:
The second, adopted in the mid-1970s, is the "Exciting" cover. The font is the same as the sans-serif cover font that came into use in 1968 (cover format #8). Note that in the blurb, "All boys" has been changed to "Anyone".
A simplified version of the "Exciting" cover, with a bolder font, came into use in the early 1980s. (Scan to appear.)
Back covers of "flashlight" editions
The laminate "flashlight" covers now in use have a partial series list on the back cover. (Scan to appear.)