Reddit reviews Locomotives: The Modern Diesel and Electric Reference

so typically you'd go by number of axles, nose style, and fan and radiator position and size. however, even within a single model there can be a lot of variation because every locomotive is custom-made to some degree for the customers.

for example, these are all SD70s, but they look pretty drastically different.

this book gets into a lot of the nitty gritty of trainspotting. published in 2015 so should still encompass at least 95% of currently operating equipment.

Locomotives: The Modern Diesel and Electric Reference
This one has a good history on the progression of modern Diesel locomotives in North America.

The Railroad: What It Is, What It Does
Borrowed this one from a co-worker many years ago. Pretty sure it had quite a bit of engineering related details.