A Sting in the Tale, by Dave Goulson (2013).
Written by one of the lead bumblebee researchers, this book is an excellent introduction to bumblebees. It is highly recommended for anyone who has an interest in bumblebees, as it explains the life and conservation of bumblebees through charming anecdotes and tales of his adventures in research, in a way that no other publication does. Dave goes on to describe how he founded the Bumblebee Conservation Trust, and looks to the future conservation of bumblebees.
Bumblebees of North America, by Paul Williams, Robbin Thorp, Leif Richardson and Sheila Colla (2014).
This book is a must-have guide to the bumblebees of North America. It has identification charts for all 46 species, along with habitat and ecological information. It is richly illustrated, with colour photographs, diagrams and maps. A bonus is that the paperback form means that it's easily taken into the field when observing bees. It is suitable for amateur recorders, as well as those who take part in the more advanced recording using dissection.