Q. Please recommend a comprehensive wild flower reference book to identify plants
I suggest you visit the publications section of the
They've just released a book
Wildflowers of New England: Timber Press Field Guide
By Ted Elliman and New England Wild Flower Society (February 2016). While we don't have this book in our library (yet), the New England Wildflower Society and Timber press consistantly put out excellent publications.
We do have the other book listed on this page,
Flora Novae Angliae, a Manual for the Identification of Native and Naturalized Higher Vascular Plants of New England by Arthur Haines. This is not a field guide. Rather, it is a reference work weighing in at 900+ pages. My sense is that the Elliman book would be a better choice bacause it is a field guide that can be easily carried and consulted on walks.
Hope this helps, and what a thoughtful gift!