Cheats of Math Application Problems in Grade 2-6 of Primary School
Cheats of Mathematical Calculation in Primary School 1-6 Grade
Elementary school mathematics thinking enlightenment 1-2 grade
These are all learned and come up with, not bad.
This set of books on mathematical thinking is a bit expensive (it is about Olympiad), and the prices of other series are reasonable.
The most important thing to read is the cheats of primary school math application problems and primary school math calculation cheats, which I think are very good.
There is also a set of old primary school mathematics books compiled by Liu, which is also very classic. But the explanation is not very detailed, and it is not easy to find.