“Beware the man of a single book.”
Hominem unius libri timeo. / Timeo hominem unius libri.
As quoted by Leonard Sweet, The Greatest Story Never Told http://books.google.gr/books?id=KuTRcjWL91AC&dq=, section: "The Gift of Lyrics", Abingdon Press, 2012
Variant: "Beware the man of one book."
See also: Homo unius libri
Disputed
Variante: I fear the man of a single book.