Here are the Library of Congress Classification numbers for the topics:
Mammals of the world - QL75-QL766
Mammals of Poland - QL767.P6
Beaver reproduction - QL737.C3 B43
Rodent reproduction - QL751
Ecology of beavers - QL737.C3 E25
Aquatic mammals of Central Europe - QL751.E8
Fossil beavers - QE871.B42