The most significant book published on recumbent bicycles is the aptly named The Recumbent Bicycle by Gunnar Fehlau. It was originally published in German, but was translated for the US market by Out Your Backdoor Press. OYB isn’t in the book publishing business any more, but you can find used copies of it around the ‘net.
As you might guess by the title, this book says nothing about recumbent trikes, which is what most people are interested in nowadays. I don’t know of a historical and technical treatise on trikes in the style of Fehlau’s book.
There are any number of travelogues written about touring on recumbents, some by our customers, though many of these are privately published and hard to find. One author who has written prolifically about tricycle touring of all sorts is adventure writer Steve Greene. I can’t claim to have read any of his books. Once I get off work I usually want to stop thinking about tricycles. But customers of ours have mentioned these and at some point I’ll probably read his book on off-road triking.
A couple of other books worth mentioning, though they aren’t truly recumbent specific… One is Bicycling Science, by the eminent bicycle scientist David Gordon Wilson, one of the instigators of the human-powered vehicle movement of the 1970’s that launched the modern recumbent bicycle industry. Wilson, an MIT professor, led a design seminar aimed at creating a really safe bicycle, and that resulted in the prototype of the Avatar long-wheelbase recumbent. Bicycling Science covers recumbents as a part of the overall design spectrum of bicycles. It is a fairly technical book, but there is enough going on between the differential equations to keep non-technical sorts entertained. You can find out what an URB is.
Another book I’ll mention, even though it has practically no mention of recumbents, is A Social History of the Bicycle: It’s Early Life and Times in America. I mention this only because of my experience trying to interest the public in recumbents, a novel form of bicycle. See, it’s like this: if you read this book you’d get a good sense of just how odd people thought bicycles looked, and in particular how ridiculous a person riding a bicycle looked, sailing onto the scene with their feet spinning around in a goofy sort of parody of walking. Men would shout curses, children would chase them, ladies would look away. It was just wrong. Then a few generations went by and everyone got used to them. Everyone rides them. Presidents of the United States ride them. They are the greatest invention of all times in terms of what you can do with so little. And now someone sails onto the scene doing exactly the same thing only rotated back 90 degrees into a recumbent position and OH MY GOD THAT IS THE MOST RIDICULOUS THING IN THE WORLD YOU’D NEVER CATCH ME ON ONE OF THOSE YADA YADA YADA. Some things never change.