"The New Book of Knowledge"
A recent edition of Grolier's premier printed encyclopedia in good condition can easily fetch over $100, with the most recent editions being worth the most money. Even an older edition, such as from the 1970s, can be worth over $100 if it's in mint condition. A very old edition, from the 1950s or older, can be worth considerably more than a recent edition --- perhaps upwards of $500 --- if it is in good enough condition to be considered a collector's item.
Grolier encyclopedias are also available in multiple electronic forms. Since Grolier abandoned CD-ROMs in favor of Grolier Online, some CD-ROM editions have become collector's items. Generally, the older the software the more it is worth. The most valuable software program may be "Grolier's KnowledgeDisc," the self-proclaimed World's First Laser Videodisc Encyclopedia. Even though all software is now outdated, it still must be in good, usable condition to be worth anything.
The Grolier Encyclopedia also came in special editions. The printed "Grolier Student Encyclopedia of Endangered Species" from 1994 in good condition is typically worth about as much as a 1994 "New Book of Knowledge" in similar condition. The CD-ROM "Grolier Multimedia Encyclopedia of Science Fiction," from 2002, considered a rare and highly useful item, can be worth over $50 if in working condition.
Grolier also publishes the "Encyclopedia Americana," the second-largest encyclopedia in English. Like "The New Book of Knowledge," a very new or very old edition in good shape will be worth the most. Generally speaking, these are worth more than editions of "The New Book of Knowledge" from the same year --- a full set can fetch $500 or more.