I love science, and I already had a couple entomology-related field guides for fun, so I thought I would expand on the collection:

3 Books on Insect Identification

2 Books on Arachnid Identification

2 Books on Tree Identification

1 Book on Cloud/Weather Identification

I Book on Bird Identification

1 Book on Faroese Flora Identification

5+ Books on Geology-related Identification

... some others as well.