Originally a “made for TV” movie, it was a surprise hit that stimulated several direct sequels. It also spun off a TV series “The Librarians”, but without Newhart. I’ve only seen a few of the series, which involves a group of “apprentice” librarians-in-training, in which Noah Wylie makes the occasional appearance. Newhart may had done a cameo or two, but not in the ones I watched.
Was Curtain in any but the first? Really enjoyed the one seen - much more than a similar movie, “Night at the Museum” (though this one did have its moments).