LONDON, March 15 (Xinhua) -- Christie's plans to auction a collection of letters, postcards and photographs of German scientist Albert Einstein in early May, according to local media.

The assortment has been preserved by Einstein's younger sister Maria Einstein, among which letters written by the scientist to Maria are considered most valuable.

"He says some things (in the letters) that I've never seen him say anywhere else, and I've catalogued many hundreds of his letters," Thomas Venning at Christie's was quoted as saying by the Guardian.

The letters can be seen as "unpublished snapshots of Einstein," which according to Venning showed "his private face" rather than the low-key and humble image that the public is familiar with.

In a letter valued by Christie's from 6,000 to 9,000 British pounds (8,386 to 12,579 U.S. dollars), Einstein wrote to Maria: "I am becoming very much loved and even more envied; there's nothing to be done about it."

In several other rare candid and self-reflecting expressions, Einstein lamented "the brain gradually goes off with age," or acknowledged his achievements had "become part of the foundations of our science."

A previously unknown photograph of Einstein as a five-year-old and the only surviving letter written by Einstein to his father are also among the auction lots.

With dates ranging from 1897 to 1951, many of the lots have not been published before.

The letters will be publicly exhibited on April 18-20, while the online auction is arranged for May 2-9.