A collection of letters, photos and more from the late Tony Bennett will go up for sale via Julien’s Auctions on April 18. The trove of items includes rarities addressed to Bennett from stars like Lady Gaga, Amy Winehouse, Madonna, Martin Scorsese, Robert De Niro, Quincy Jones and Barbra Streisand, among others.

The priciest item - an estimated $20,000 to $30,000 - is a typewritten thank you letter from Martin Luther King Jr. to Bennett dated April 5, 1965. In it, King talks about the historic Selma civil rights march to Montgomery and thanks Bennett for singing for the marchers and, mentioning his “good friend, Harry Belafonte” who told King of the difficulty Bennett had in rearranging his schedule to perform.