"Text fly within the book only."  At first, we interpreted that as, "words take flight within the confines of the book covers."  Given that the book in question (scanned here by Archive.org) is about Merlin, that felt about right.  (Even so, one might argue otherwise, that text can fly outside of a book as well; consider some persons being "crammed to the throat with Scripture, so that they cannot open their mouths but a text flies out" [An Inquiry Into the Nature of Responsibility by T. Simmons Mackintosh, 1840]).  But then we realized that a "text fly" is what emerges from a bookworm's cocoon ... and so this particular book has encased or imprisoned a text fly, presumably for entomologists.