I teach fifth grade in a small K-5 school. This will be my fourth year of using Investigations. My question is: which books should I use each year and which books are optional? It's impossible to use all the books for any grade. I've used all the books for grade 5 with the exception of the book on probability and the fifth grade book on change (I did use the fourth grade book on change this past year). I always use the Building on Numbers You Know, the book on fractions, percents and decimals, the book on measurement and a geometry book. But which geometry book--2D or 3D? Is the intro. book necessary? I'd love to hear what other fifth grade teachers are doing.