Q. My textbook cites another (CSS library) book I am a distance student. Could I ask a librarian to check that book for it's sources?

It looks like the book you are interested in is this -

Carter, R, Aldridge, S., Page, M., & Parker, S. (2009). The Human Brain Book. New York: DK.

It is a young adult book.

I believe this is the passage being referred to:

"At around three years, the hippocampus matures, allowing the retention of memories. Few memories can be recalled from before this time" (p. 203).

DK publishes gorgeous, illustrated young adults book. Since this book is not intended to be scholarly, there is no direct attribution to this information.