I'll start with Walter Stager's Tall Bearded Iris, A Flower of Song, from 1922. This book is packed with a thorough examination of the iris in story and song, with numerous quotes and citations. What a wonderful collection for those who enjoy literature, art, music and iris history - all rolled into one. This book is a singular work whose like has not been matched in all the decades since. There is always something to explore again in its pages.
Next is Rainbow Fragments, by J. Marion Shull, from 1931. This is one of the earliest books to contain color plates of iris flowers, along with many black and white photos. Mr. Shull was a noted irisarian and was very much into writing about the best irises of his day. This book contains many descriptions of selected varieties, along with some iris history, cultural and hybridizing information. It was one of the best books of its age and is a fun read too. Altho Mr. Shull has a very flowery and poetic way with words that is amusing from a modern perspective, it is full of great information and stories about the early 20th century hybridizers and irises.
In 1954 William J. McKee and Prof. J.R. Harrison published their book Half Century of Iris. This book, being a comprehensive history of iris development during the first half of the 20th century, is invaluable to the historic iris collector. Numerous histories of patterns and color development are accompanied by varietal family trees and biographical info. It was aimed at the hybridizer of the 50's looking to explore the future trends of development. This is one of my favorite books to just pick up and open at random to see what it is discussing. More often than not you'll see line breeding mentioned, as the merits of a scientific focus were the theme of the day and these authors thought highly of this method of advancement.
If you want something that is purely a delight to read for its easy, breezy and fun style find a copy of English hybridizer Harry Randall's Irises, from 1969. What a wonderful addition to your iris library this one is with so many stories and anecdotes about the history of irises after WWII in England and the US. Good advice is there too, but more than anything you'll enjoy the history and the forthright perspective of this master hybridizer during magical time in iris development. Mr. Randall's obituary in the AIS Bulletin by George Waters informs us:
It was Harry's practice to invite a few members of the BIS to his home in Beaconsfield to hear him read recently completed chapters of his book in progress. The guests were expected to offer criticism of the work, but Harry's formidable reputation tended to inhibit comment. My own attempts to meet his wishes on these occasions earned the gift of a carload of recently dug rhizomes. Harry's manuscript was almost complete when he died, and, with final details attended to by Mrs. Randall, George Preston, and other friends, Harry's Irises was published posthumously.
There are numerous books about irises produced from this time forward, but I think nothing has yet come out of interest to those into iris history to match the brilliant addition to the canon by Clarence Mahan in his 2007 book Classic Irises and the Men and Women Who Created Them. Here is a book I go back to time and time again. There is always something new in its pages to catch my interest and send me down a new path of discovery. This amazing work greatly expands our knowledge of the people behind the beautiful flowers we now treasure.
While these books are now out of print, it is fairly easy to find them offered at Amazon.com, Alibris, Abe's Books or other online book retailers. Sometimes hunting them down is almost as much fun as reading them. I hope you'll explore some of them and help keep the living memory of our favorite flower alive.