The Spanish word 'librillo' comes from combining the Spanish word 'libro' (meaning 'book') with the diminutive suffix '-illo'. The base word 'libro' itself derives from the Latin word 'liber', which also meant 'book'. When the diminutive suffix '-illo' is added to 'libro', it creates the meaning of 'small book' or 'booklet'.

This is a classic example of Spanish word formation using diminutive suffixes, where '-illo' is one of several common diminutive endings (along with '-ito' and '-ico') that can be added to nouns to indicate smallness or endearment.

The most obvious related Spanish word is 'libro' (book), which is the base word from which 'librillo' is formed. Other common related Spanish words include 'librería' (bookstore), 'librero' (bookseller or bookshelf), and 'libreto' (libretto, a small book containing the text of an opera).

Several English words share the same Latin root 'liber', including 'library', 'librarian', and 'libretto'. The connection is clear when you consider that all these words relate to books and written materials. The word 'liberty' also comes from this Latin root, as 'liber' had a second meaning in Latin of 'free' - this gives us an interesting etymological connection between books and freedom in both languages.