One of the top tricks to stay organized is decluttering. When doing that, you will decide to keep the item, donate it, or throw it away. It's a good practice to keep a basket for each category with a tag on them so that you can ask your family members to categorize stuff regularly.
To avoid buying too many things that you won't keep, try to borrow first. One example is the book. Borrow from the library or get an electronic version unless you want to read it repeatedly in the paper.