Children's book, journey to new planet, butterfly-like aliens

This is a children's book that I read in about 1991, although I suspect it may have been somewhat older than that (it seemed dated at the time). The main characters leave Earth in a spaceship to find a new home, for reasons that escape me now. They are told that they can only bring one book each, and hence have to read it slowly, reading every sentence twice. The narrator, a small child, notes that the computers of more advanced spacecraft could hold many hundreds of books, but their spaceship is old, and does not have this capacity. When they finally reach the planet, they find it is inhabited by man-size butterfly-like aliens, that they eventually befriend. I seem to recall that the aliens shed their skin, and the Earth colonists were able to use these to make clothes and blankets. In the end, it transpires that the book the narrator brought with him (her?) was a blank notebook, in which he/she has made a record of the first year of the colony (which is presumably supposed to be this book).

"The Green Book" by Jill Paton Walsh.

http://www.allreaders.com/topics/info_29145.asp

Thank you so much, that's bothered me on and off for years!

Thanks bookel!