I have to say this right off the bat, because I think it is funny and potentially points to my birding ignorance. At the end of this Sibley guide there is a "Species Checklist" which makes a lot of sense until you consider the size and weight of the book. Surely Elphrick, Dunning and Sibley don't want you carrying this behemoth into the field!
What you need to know about this guide is that it is unique in its voice. In addition to its glossary there is a section on author biographies because each chapter is an essay written by a variety of folks. Not just ornithologists and other types of bird experts contributed an essay or two. Students, biologists, naturalists, ecologists, writers, museum curators, photographers, and even a B&B owner.
Additionally, the illustrations and maps are stunning. ( )

Such a great guide to birds! My father is a teacher of biology and ornithology (and is often referred to as 'the bird man'). As an expert on birds and the birding world, he has a handful of books that are his 'go-to' books for birding information, and Sibley is always a go-to. ( )

a reference book for adults

This is one heck of a book. The research and technical research is very good but not overpowering to ordinary lay person. A beautiful and readable book for anyone being introduced to birding and a good "polisher" for the long-time birder. ( )