The first full-fledged book by P. L. Travers was Moscow Excursion, a 100-page travelogue published in 1934 in the U.K. and in the U.S.A. and never reprinted ever since, as far as I know.
A blogger named Scott quotes the book in his Furrowed Middlebrow Weblog:
Every possible rule was broken, the text was murderously cut about and great wads of Erasmus and anonymous buffoonery interpolated. The characters, too, were altered. Rosencrantz and Guildenstern became a couple of clowns who were let loose before a drop-curtain every time a scene needed to be changed. But, when you come to think of it, these two are rather vaudeville and are easily turned into slapstick comedians. How often have we groaned when some star actor rhetorically hurls at the empty air the question whether it is nobler in the mind—etc, etc, and not even echo makes reply. Not so here. The speech was divided between Hamlet and Horatio. The two students are in the library of the palace, Hamlet turning a globe, Horatio on steps reaching up to a high shelf for a book.
To be or not to be—begins Hamlet.
That is the question, returns Horatio, as one who observes, Boy, you've said it. And so the speech goes on and for once appears real and the natural comments of very young undergraduates.
They seem to have invented Stoppard then, in 1934, there, in Moscow.
Besides, there is an interesting article with some subtle comments on the book: Mary Poppins and the Soviet Pilgrimage: P. L. Travers’s Moscow Excursion (1934) by John McNair of the University of Queensland.
And they are planning for whatever reason to publish Russian translation of the book this autumn, an excerpt having been published in an obscure magazine.