Happy New Year!
I bought the Kaufman Field Guide to Butterflies of North America last summer in New Hampshire. The previous books I'd bought two years earlier when I was very new to butterflies were regional and didn't show both upper and under sides, I needed something that did. The local bookstore had a copy, Amazon reviews were positive and it was commended in Mariposa Road, my "holiday reading".
It got plenty of use, particularly as I read Mariposa Road and wanted to see what Bob Pyle was referring too.
I did take it out on field trips, bit heavy (1lb?) but will fit in a cargo pocket for short walks.
It doesn't help with identifying caterpillars but does include larval foodplants for each species.
The range maps were moderately useful, a bit small but good enough to eliminate "out of state" possibilities
You can "Look Inside" on amazon.com but not on amazon.co.uk to get an idea of the style and content.
I assume you're headed across the pond? I filed my sightings on
http://e-butterfly.org
Hope that helps, I'll try to answer any more specific questions you may have.
Kelvin Richards
PS My first post to UKB...that's one 2014 resolution that I can now tick, I've been lurking here for too long and it's time to contribute!