If my love to you can't be said in a posy, it can't be said in a library.

The line contrasts a “posy” (a small bouquet, and by extension a brief, conventional token or short verse) with a “library” (the vast accumulation of written language). The speaker claims that if love cannot be expressed in the simplest, most concentrated form, then no amount of elaboration or literary abundance will succeed either. It elevates sincerity and aptness over verbosity: true feeling should be communicable in a small, fitting gesture or a few right words, and if it is not, the failure is not one of scale but of essence. The aphorism also gently mocks overwrought romantic rhetoric, implying that love is proved by clarity and immediacy rather than grand textual performance.