I'm quoted on the back of the dust cover as writing, in another review: "I would say that this is a book which should be on the shelf of every serious American (and other English-speaking) Esperantist, and I cannot recommend it highly enough for those who want to move from the status of beginner to that of expert."
I'll stand by that, but go a step further: Despite this book's basic purpose (to help the English-speaking reader understand more about Esperanto), and despite the fact that it's definitely not a textbook, I would say that it's actually possible to learn the language from scratch via the first half of this book. Not for everybody, of course, but for the committed individual who is willing to plow through 118 pages of great (though never excruciating!) detail about Esperanto, its grammar and its word-formation system without drills or exercises. The fact that Jordan has a terribly dry sense of humor, which he doesn't hesitate to share with the reader, makes this relatively painless. By the time you reach the end of that first section, you should have a full command of the language, except, perhaps, for vocabulary (and I use the word "perhaps" advisedly, because at least half of those 118 pages are given over to a multitude of examples of usage, and you should be able to pick up a pretty good vocabulary just from those).
Oh, and if you want to increase your vocabulary beyond this (and you should), don't forget to read the second half of the book, "Potentially Troublesome Words," in which you'll find much more detail about those words in Esperanto that can mislead the English-speaking student, as well as the words you ought to use instead.
Note: Jordan's (not overlong) lists of transitive and intransitive verbs that will cause trouble for English-speakers (because in English the transitive and intransitive versions of the verb are identical) should be studied and memorized by every American student of the language. Read pages 77-79 and you'll be sitting pretty.