Lighthouses and lost books

Living as I do in close proximity to a lighthouse, with its beam flashing out as a warning to ships not to stray too close to this dangerous coastline, I often have cause to remember a family who bought a holiday home in Nantucket in the early 20s. They were the Gilbreths - father and mother Frank and Lillie - who pioneered time and motion study. They had twelve children and Frank Gilbreth, when asked why he had such a large family, would pretend to think a while and then say it was because they came cheaper by the dozen. The holiday home consisted of a small building and two bug-lights (small lighthouses which, when they lined up in the sights of a ship's captain, would lead him safely through a channel into the harbour - much like the beacons of Knysna lagoon). The children used the lighthouses as dormitories and the two books they wrote about the family later in life, Cheaper by the Dozen and Belles on their Toes, told the tale of growing up in such a large and interesting family. By all accounts, the father was a real character who developed his time and motion theories through such methods as timing his children on how long they were in the bathroom, etc. With so many of them, it would have been quite a logistical nightmare in the days of single bathrooms! (The Von Trapp family were doing similar things on the other side of the Atlantic at about that time, although apparently on a larger budget!)

I read these books over and over as a child, as they were on my mother's bookshelf, and although I have searched and searched, I cannot find either book. Even stranger, no one seems to know what I am talking about when I ask where they are! If anyone knows where I can find these books, please let me know. I would imagine they are out of print now, although a visit to a bookshop could give me the answer to that!