Most young readers love a mystery, whether it is a serious one involving a life-threatening situation or a simple one that revolves around an everyday problem. The following two novels are bound to appeal.

The first takes a raucous lope through a school year as a group of fifth-graders gets involved in purchasing a new drinking fountain. The other provides a backdrop for prejudice and intrigue when two students - one black, the other white - find that their parents were personally involved with each other in high school years before. Both are good reads for students in grades five and up.REGARDING THE FOUNTAIN: A Tale, in Letters of Liars and Leaks (Kate Klise. Illustrated by M. Sarah Klise 138 pages. Avon Books. $14). The story begins when the school principal, Wally Russ, slips in a puddle beside the drinking fountain. He decides to replace the leaky fountain and sees it as an opportunity for the school's fifth-graders to learn some lessons in civics through purchasing and installing the fountain. Little does he realize that his request for a new fountain has gone to the wrong person: Flo Waters designs public fountains, not the simple drinking ones appropriate for a school. Flo jumps at the chance to provide the fountain, and the fifth-graders help her design it.

As Flo travels around the world, she sends the children gifts - band instruments, costumes from different countries, a pair of monkeys, recording instruments for their projects and many ideas for the drinking fountain. The children, in turn, invite her to "Meet Dry Creek Day," a celebration of the city, at which they read their history of the town, which has lost its water supply.

In actuality, there is a water supply, but it is owned by Dee Ell and Sally Mander (say those names fast, and you get the picture).

The fifth-grade sleuths discover that Dee Ell and Sally Mander have capped the natural springs of Dry Creek, and its head is - you guessed it - the drinking fountain in the school.

This clever story is made more so by the unusual style: letters, memos, newspaper clippings, postcards and receipts enhanced with pen-and-ink sketches on every page. The quirky characterizations of Wally Russ (Flo Waters actually sends him a live walrus to keep him company), Sam N., Dee Ell, Sally Mander and Barry Cuda, the school attorney, add to the humor of the whole escapade.

The plot moves at breakneck speed, and there's no doubt young readers will giggle their way through the whole thing.

The author from Norwood, Mo., and artist from Berkeley, Calif., are sisters. This is their first book for young readers.

LIVES OF OUR OWN (Lorri Hewett. 214 pages. Dutton. $15.99). "It's hard to see each other at school and act like we're just acquaintances even though we're more than that . . . We decided we wouldn't hide our feelings for each other anymore . . . As we thought about the Old South Ball, we realized that maybe the issue isn't as black and white as it seems . . . Why can't the ball become a school event?"

The letter appears in the school paper, sneaked in by Shauna because she believes the editorial staff would not have approved it.

Shauna has moved with her father to his hometown and attends an "integrated" school after her parents' divorce. There she finds prejudice as she's never known before, such as the Old South Ball, attended through years of custom only by the white students. Because she writes an editorial about inclusion of all students and she allows the letter from a mixed-race couple to be printed, she is ostracized by the popular girls' club and by white student Kari Lang.

Shauna discovers that her black father and Kari's white mother knew each other years before in high school and each had been mysteriously sent away during their senior year.

In an almost too-perfect problem novel, Shauna and Kari team up to find out about their parents and, while doing so, find out their cultural similarities override the differences.

The social problems in "Lives of Our Own" would be a good basis for discussion of integration and prejudice in today's schools and society.