This year marks the 40th birthday of the most famous 10-year-old crime solver. Encyclopedia Brown, ''America's Sherlock Holmes in sneakers,'' has been solving head-scratchers in Idaville since 1963, and he's at it again this month in his 23rd book, ''Encyclopedia Brown and the Case of the Jumping Frogs'' (Delacorte, $14.95). And here's a twist you didn't expect: Encyclopedia's creator, Donald J. Sobol, isn't a front for a brain trust of writers, à la the Hardy Boys' ''Franklin W. Dixon'' or Nancy Drew's ''Carolyn Keene.''
All the books were written by Sobol, 79, a former newspaperman from the Bronx now living in southern Florida. ''I don't sit down at a typewriter and say 'I'm gonna write a story today' and dream it up,'' explains Sobol, whose ''Jumping Frogs'' cases are as tough to crack as ever. ''I look at my card file, and I'm constantly reading for background material.''
But Sobol doesn't read mysteries! ''What I don't want to do is take somebody else's idea,'' he says. ''That's a trap in mystery writing.'' Like many of his young fans, he prefers short stories. ''I started reading Harry Potter and I thought it was good, but something came up and I forgot it,'' says the author of roughly 300 Encyclopedia Brown cases. ''Pretty soon I'll pick up a Harry Potter again. But not the 700-page one. I don't know how she does it!''