An unpublished diary of Salvador Dalí, its pages swarming with images of contorted nude figures and naked women being carried off by galloping horses – and columns of expenses calculations – covering the period when he fell in love with, seduced and eventually married the wife of one of his best friends and fellow artists, is to be auctioned as part of a vast private collection covering the intertwined lives of some of the most famous dada and surrealist artists.

The diary, to be auctioned next week by Sotheby’s in Paris with an estimate of up to €50,000, is among 500 lots including 20 letters and sketches from Picasso, one on the back of a restaurant menu, and illustrated letters, photographs and poems by André Breton, René Magritte, Joan Miró, Max Ernst and Roland Penrose and many others, taking the expected auction total to more than €4m.

A Picasso drawing on the back of a postcard. Photograph: WestImage - Art Digital Studio/Sotheby's

One jaunty holiday postcard from Picasso sent to the poet Paul Éluard in Paris, signed by a group including his lover Dora Maar, reads “Ici on aime ses amis” (“here we love our friends”) – which was certainly true. Paul and Gala Éluard had previously lived in a ménage á trois with Ernst, who designed Éluard’s book plate with the motto apres moi le sommeil (after me only sleep), before Gala left them both for Dali.

The friendships endured: the collection includes Ernst’s portrait of Éluard, and his 21 sketches of Gala, as well as a book given by Éluard to Gala and Dali after their marriage, signed by him and Man Ray “de tout notre coeur” (with all our heart). Gala and Dalí remained married, despite many affairs, until her death in 1987.

An original Miró watercolour which is part of the auction. Photograph: WestImage - Art Digital Studio/Sotheby's

In other letters, Picasso discusses his complex relationships. One, partly written by Maar, begins rapturously: “Ah, qu’il est doux d’aimer” (ah, how sweet it is to love).

The collection was built up over decades by the owner and his son, and is regarded as the best in private hands. It will be sold on Tuesday.