I’m an oldest child, but my mother was a middle child and growing up loved Jane the Middle Moffat by Eleanor Estes (and all the other Moffat books.) I still think it is a peculiarly good insight into the mind, and the value, of a child with older and younger siblings in a poor family.

https://www.amazon.com/Middle-Moffat-Eleanor-Estes/dp/015202...