I came across this book, Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping, at Amazon, which was excellently reviewed by people who have read the book.

An excerpt from an editorial review:

Ogren, a former landscape gardening instructor, proposes that in our private and public landscaping we need to reduce the use of plants that cause allergies. Until the publication of this book, the information needed to make allergy-free choices had not been compiled in one easily accessible source; nor had a scale been devised for rating plants that cause allergies. Here, Ogren accomplishes both these aims. [snip] As David Stadtner, a practicing allergist, writes in his foreword, "the book should be in the library of every nursery and municipal park department."

I personally haven't read it, but I'm thinking of getting one, after reading all the comments. Out of 28 reviews, all but one gave the book the maximum score of five stars! This must really be a good read!