I don't know what books you checked, but if you study a certain kind of math, it's only slightly less important than knowing the alphabet. Try a book with "analysis" in the title. (For example Friedman. I can assure you that even though the notation is only used a few times on the first few pages, you would use it all the time when you work through proofs and do problems). You should find it in any book on linear algebra as well. How else does the book define a subset before it asks you to determine if it's a subspace?