Opus 100, by Isaac Asimov. New York: Dell Books. $2.25; Opus 200 by Isaac Asimov. New York: Dell Books. $2.50.

Excerpts from Asimov's first and second hundred books -- "Opus 100' and Opus 200," respectively -- are chock-full of expert-sounding information, from biblical lore to physics, literature, robots, and, of course, science fiction. The stories are tied together by Asimov's disarmingly frank, though not always modest introductions, such as his admission about mathematics: "It takes a lot of ingenuity for me to write about mathematics, since I know so little about it."

One may not be fascinated by every bit of knowledge, but one can't help finding some interesting morsels, such as the derivation of the word "mob" (Latin mobile vulgusm -- fickle multitude); theories on black holes; or the extinction of the great auk in 1844. If nothing else, the books are marvelous samplers of the variety of Asimov's work.