I say informative instead of information because information is what happens when someone becomes informed--takes a sign, in the semiotic sense, to be "about" some part of the tangible or intangible world, and to do so in a way that one learns. Information does not happen when you re-read yesterday's news reports unless you see or understand something you did not notice before. Informative indicates that the source has the reasonable potential to inform someone, sometime.
The only element of a book in 1931 that is not addressed in this definition is the matter of length. The institution of coherent, book-length discourses is important to human society and to our intellectual lives, but length is not a central concern for library practice outside of the provision of shelf or file space. So I am comfortable with thinking of articles and other short sources as books so far as "books [informative sources] are for use."