The phrase 'sibi librum emit' translates to 'he/she buys a book for himself/herself,' highlighting the use of reflexive pronouns in Latin. In this context, 'sibi' is a reflexive pronoun indicating that the action of buying is done for the subject's own benefit, which showcases an important function of reflexive pronouns in expressing relationships between subjects and their actions. This construction emphasizes personal interest or benefit in the action being described.