QUESTIONS COME to the Library from all over -- not just about scholarly pursuits, but for advice on how to waterproof the sidewalk, raise rabbits, hunt ducks and fly the flag. Especially every four years, correspondents want to know which Bible passages different Presidents used for their Inaugural Address.

Many requests are referred to the supplicant's state library. But for the hundred or so most-asked questions, the Library has developed form letters in reply.

Question: Was German ever the official language of the United States? Answer: No, it never came close, even in Pennsylvania Dutch country. The Library's full and patient reply takes two typewritten pages, single-spaced. Having first received such inquiries in the late 1930s, the Library attributes the myth to Nazi propaganda.

Question: Does the Library own a letter from Pontius Pilate to Tiberius Caesar concerning Jesus Christ? Answer: No. Your sources are spurious. No such missive ever existed.

But if it did, it would probably be here. A SERVICE INDUSTRY

The local branch of your county library can answer a surprising number of quick questions over the phone, such as how long the Nile is and who the Supreme Court justices are. When calling your library, ask for the ready- reference division.

But maybe you've gone to the local library with a bigger thirst and come up dry after looking through its books. If you then decide to write to the Library of Congress, be as specific with your question as you can, and tell them what sources you've already checked. Write to: General Reading Rooms Division, Library of Congress, Washington, D.C. 20540.