Reddit mentions of The Insects: An Outline of Entomology

The Insects: An Outline of Entomology by Gullan & Cranston is pretty much the standard textbook on entomology.

There are two main textbooks that I've seen used:

• The Insects: An Outline of Entomology by Gullan
  
  
• Introduction to the Study of Insects by Triplehorn and Borror

There is no single answer to the field guide question - you should just look for one relevant to your area. Conservative estimates put the number of insect species in the world around six million, so no single field guide is going to be able to tell you more than very common species for a specific area and maybe some family-level keys to give you a general idea.

As far as textbooks go this one is pretty decent. If you are in a university there's a good chance it's in your library, otherwise it's not super expensive 2nd hand.

I'd recommend Gullan and Cranston's The Insects: An Outline of Entomology.

A field guide is fairly regional, so you'll need to let us know where you're located!