The author does offer a brief description quoted from Caffe Molinari linked here, but as a coffee newbie much of what is being described escapes me. Here is the quoted text from Braithwaite's preface:
“Café Allongé, also called Espresso Lungo, is a drink midway between an Espresso and Americano in strength. There are two different ways to make it. The first, and the one I prefer, is to add a small amount of hot water to a double or quadruple Espresso Ristretto. Like adding a splash of water to whiskey, the small dilution releases more of the complex flavours in the mouth.
“The second way is to pull an extra long double shot of Espresso. This achieves approximately the same ratio of oils to water as the dilution method, but also releases a different mix of flavours due to the longer extraction. Some complain that the long pull is more bitter and detracts from the best character of the coffee, others feel it releases even more complexity.
“The important thing is that neither method of preparation should use so much water as to result in a sickly, pale ghost of Espresso. Moderation in all things.”