The 13th Floor by Sid Fleishman
In attempting to find his sister Liz, who has disappeared, Buddy steps off the elevator onto the 13th floor of the old building and finds himself 300 years in the past on a pirate ship. And it just happens to be a ship belonging to an infamous ancestor, Captain Crackstone who according to family history was hanged for piracy. When the ship is set upon by other pirates in search of treasure, Buddy and the captain are set adrift in a small boat. Fortunately, they are washed ashore in New England, but only to face new problems. Another of Buddy's ancestors, 10-year-old Abigail, is being charged with witchcraft and Buddy's sister Liz, who happens to be a lawyer is defending her. Unfortunately, Liz is a little too modern and ends up under suspicion of witchcraft herself. Somehow, they need to save Abigail, save Captain Crackstone, and get themselves home, preferably with some of the Captain's treasure.
There's lots of action in this fast-paced story, but it's all kept pretty light-hearted with Fleishman managing to sucessfully juggle all these sub-stories and neatly wrap everything up in a little over 200 pages.