A fabulous book about Britain's Butterflies. It covers current resident and migrant species as well as rarer vagrants and presumed extinct species. Each species has loads of information about life stages, larval food plants, favoured nectar sources, ID tips, habitat, distribution, time of year to see them, as well as great photographs. While I think it's slightly large for a carry-along field guide, the cover is plastic wrapped to protect it from the rain.
I volunteer with Butterfly Conservation in Warwickshire and this will be invaluable as I learn more about butterflies, their habitats, and protecting both.