The following, according to Encylopedia Buying Guide, are among the nation's most popular encyclopedias for adults and young adults. The prices quoted do not include shipping charges.

Academic American--Age 12 to adult. Published by Grolier. Cost is $447.75 for 21-volume set, including index, payable at $9.95 for 45 months. Sold by phone. Also available "on-line" to Dow Jones, BRS, and CompuServe computer subscribers. Known as a "short entry set": few entries are longer than 750 words. Call 800-243-7256.

Americana--14 to adult. Published by Grolier. Sold in the home. Price: $960 for the 30-volume set. According to the editorial division, articles are pitched to the reading level of those most likely to look up the subject. (The article on "Mouse," for example, is written on an elementary level, that on "radioactivity," more sophisticated.) 585-9279.

Britannica--15 to adult. Published by Britannica. Prices start at $1,099. Salespeople come to the home at the request of prospective customers. The New Britannica-Three includes: a one volume "Outline of Knowledge"; a 10-volume "Ready Reference" providing short entries of 750 words or less, aimed at younger users; a 19-volume in-depth "Macropedia" covering subjects in more depth. 790-0560.

Colliers--High-school students and adults. Published by Macmillan. Cost: $899.50 for 24-volume set. Sold in the home. Articles vary in length. 212-702-2000.

Comptons--9-18. Published by Britannica. Cost of 26-volume set: $549. Salespeople come to the home. Extensive use of color illustrations, graphics and maps. 312-347-7390.

Merit Students--9-18. Published by Macmillan. Cost: $899.50 for 20-volume set including index. Combines long and short entries. 212-702-2000.

World Book--9-18. Published by World Book in Chicago. Least expensive set: $499. Salespeople come to the home. The 22-volume set includes an index. World Book makes use of extensive color and graphics, and does reader testing in sample classrooms around the country. 356-6877.