A prolific writer, he was instrumental in unifying the taxonomic knowledge of the plants of North America. Of Gray's many works on botany, the most popular was his "Manual of the Botany of the Northern United States, from New England to Wisconsin and South to Ohio and Pennsylvania Inclusive", known today simply as "Gray's Manual". Gray was the sole author of the first five editions of the book and co-author of the sixth, with botanical illustrations by Isaac Sprague. Further editions have been published, and it remains a standard in the field. Gray also worked extensively on a phenomenon that is now called the "Asa Gray disjunction", namely, the surprising morphological similarities between many eastern Asian and eastern North American plants. Several structures, geographic features, and plants have been named after Gray.