There's much to say about this little book. Since the creation of Volapük by Johann Martin Schleyer in 1880, there have been more international auxiliary languages than one could count, so many that it's impossible for all of them to have ever gained a sizable speaker base. Novial is unique in that it was the only one designed by a qualified linguist, but its provenance means little to us now. Like its contemporaries, Novial never survived beyond the Second World War, and so far it seems unlikely it'll ever be revived, though certainly some have tried. It is a dead language with dated ideas and scarcely anything to speak in its favor, but that doesn't mean this book in and of itself is useless. It's anything but.
An International Language is just as much a brief history on interlinguistics as it is a Novial grammar. A surprisingly lot is covered here, though much of the detail is hidden in passing when Jespersen discusses a particular word choice or explains a design decision that stands his creation apart from its competitors. The first part of the book I'd say is recommended reading for anyone curious about international auxiliary languages, especially those of this period. Jespersen, though fiercely opinionated, gives a strong introduction of the players of the time, especially ones that remain as obscure as Novial is today, such as Idiom Neutral. Much of the book revolves around how other languages have attempted to resolve X problem, and how Novial attempts to position itself ahead of the pack. See: the chapter on grammatical number, which begins not with how Novial nouns form the plural but instead on how the system works in Esperanto.
This is fascinating and yet a critical misstep from a pedagogical perspective, as it buries any and all information about Novial—the language we're here to learn—in a heap of unrelated material. There is no index to be found, or really any way to sort through the book beyond the chapter titles. As learning material for the nascent language, it is convoluted to the point that the book fails as a reference despite being more or less complete. What a shame. It's difficult, if not impossible, to pinpoint why an IAL lives or dies, but if there's ever a reason for why Novial has failed to see a revival in recent years, maybe it begins with the source text being so unfocused.
Dieses Buch scheint mehr eine Ansammlung von dem zu sein, was Novial alles *nicht* sein soll. Manchmal war ich etwas verwirrt, was Jespersen denn nun eigentlich vorschlägt. Vorkenntnisse in mehreren IALs (Esperanto, Ido, Occidental...) ist oft notwendig, um die Argumentation nachvollziehen zu können. Die meisten Beispiele sind nicht übersetzt, anscheinend war Jespersen sehr überzeugt von den sprachlichen Fähigkeiten seiner Leser.