Did you know that famed Rabbi Menahem Mendel of Kotsk spent the last 20 years of his life locked in his room, refusing all visitors?

Or that biblical-era rabbis categorized symptoms of insanity as “wandering out alone at night, remaining overnight in a cemetery” or “tearing one’s clothes”?

And did you know the Lubavicher Rebbe served as electrical engineer in the U.S. Navy during World War II?

How handy it is to have “The Student’s Encyclopedia of Judaism” within arm’s reach. The big, colorful, 390-page reference book is the kind of tome a curious reader could spend hours randomly perusing, noting entries such as sha’atnez, throne of God, the prophet Zephaniah and stopping to examine sex, naturally.

And, while we’re there, let’s talk about sex (that’s where students will look first, anyway).

Sex, it seems, is seen in the Bible as an essential part of marriage. And, with the creation of woman, man is told to “leave his father and mother and cling to his wife, so that they become one flesh.” (Gen. 2:24)

That’s as good a reason as any to get out of the house.

Furthermore, Jewish law states that sexual relations are “the wife’s right and the husband’s duty. He must fulfill this obligation at specific intervals, based on his occupation and ability.” (Ket. 61b) “Occupation” is a simple matter of working long hours or not — the ancient Israeli equivalent of a traveling salesman might not make it home every week to fulfill his “obligation.” As for the term “ability,” it is not explained at all, sadly. Finally, spouses are not permitted to think of anyone but their partners during sexual relations, as “intimacy means a total relationship between the two parties.”

The encyclopedia, aimed at students ages 12 to 18, is similarly businesslike and thorough in clarifying other aspects of Jewish law as related to concepts like “mercy,” “justice” or “life.” It does an excellent job of recounting traditions, paraphernalia and biblical figures such as the aforementioned Zephaniah, the ninth of the minor prophets (and certainly the last, alphabetically).

Far from being limited to teens, this encyclopedia could be incredibly useful to anybody who didn’t grow up in a yeshiva — or even those who did.

In fact, it might be more useful for an older reader than a younger one. Editors Geoffrey Wigoder, Fred Skolnik and Shmuel Himelstein have done an impeccable job of providing concise entries covering subjects such as the life of Maimonides, the meaning of Purim and summaries of biblical tracts.

Entries with political connotations, however, such as “Israel” and “Holocaust,” are maddeningly brief and short on facts. (The entry for “Maimonides,” for example, is slightly longer than the one for “Holocaust.”) And, in a volume replete with vibrant, glossy photos and illustrations, the image alongside “Holocaust” is a rather tame etching of a camp inmate reaching under a fence for a loaf of bread.

And it will not answer the questions about Israel that students are being confronted with both on and off campus (the only entry for “Palestine” is “Palestinian Torah”).

That being said, this is an especially useful reference, and will become a welcome fixture in the offices of j.

“The Student’s Encyclopedia of Judaism,” edited by Geoffrey Wigoder with Fred Skolnik and Shmuel Himelstein (352 pages, New York University Press, $39.95).