Carmena, the first left-wing mayor of the Spanish capital in over 20 years, was sent a Champagne guide, worth around €25 ($27), by a Spanish business owner.
Her letter in reply, in which she rejects the gift, has gone viral in Spain after being retweeted by Carmena's left-wing coalition, Ahora Madrid.
"You know, dear friend, that the authorities cannot accept gifts," wrote the mayor in the letter, dated December 21st.
Carmena added that the book would be sent to "a municipal library so that the citizens of Madrid could enjoy it".
Carmena has made a name for herself since she took office in May 2015 as a belt-tightening mayor, after giving up many of the usual privileges associated with the office in order to save money.
She gave up her official VIP seats at Madrid's iconic Las Ventas bullring, as well as opera seats and even her mayoral car, opting instead to take the metro to work.
Manuela Carmena has shunned her official car in favour of taking the metro. Photo: AFP
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She also pulled funding for Madrid's prestigious bullfighting school and has vowed to finally rid the Spanish capital of the ghosts of former dictator General Francisco Franco - by removing all associated street signs from the city.