It could only be him. James Dean is squinting and smiling, a cigarette hanging from his lip, and wearing a leather jacket and glove. The silver gelatin photo from a 1955 motorcycle session is a classic Hollywood portrait but it is also unique. It measures 13.5in x 10.25in and has an inscription, written with a dark blue ballpoint pen, from Dean to his agent Jane Deacy: “To Mom / My heart and thanks / Jim.”

The actor regarded “Lady Jane”, as Deacy was called, as a second mother because his biological mother, Mildred, died of uterine cancer when he was nine years old, and he was estranged from his stepmother.

“It’s gorgeous,” says Nate Sanders, who is auctioning the photo (bidding starts at $7,500) along with nearly 400 lots from a goldmine of Dean memorabilia discovered through Deacy’s estate. “To find James Dean 8in x 10in signed photos is rare. To find an oversized photo is super rare. This may be the best signed photo one could ever hope for. I bet it breaks records.”

In Hollywood mythology, Dean is hard to beat. Born in 1931 in Marion, Indiana, he immersed himself in theatre from a young age and landed his first onscreen acting job – earning $30 – in a Pepsi-Cola commercial in 1951. He got his big break on Broadway and was soon spotted by Hollywood director Elia Kazan. He had made just three films – East of Eden, Rebel Without a Cause and Giant – when he died in a car crash at 24.

Deacy kept her collection of Dean memorabilia, unprecedented in both size and quality, until her death in 2008 and it remained with her family. Speaking from Los Angeles, Sanders, 50, explains: “One of the relatives is getting old and frail and decided to sell it – it’s going to be great for her golden years to live out with plenty of money.”

The auctioneer, who has sold Dean and other Hollywood memorabilia in the past, flew to the home near Tysons in northern Virginia to inspect the precious items, stored in a simple cardboard box. “It was a big eye opener just to see the see the collection and see the depth of it. We all sat down on their living room floor and pieced through it and I was quite blown away because just to have one James Dean contract is amazing but his only movie contract that he’s ever signed is in that lot: East of Eden.”

The Warner Bros contract signed by Dean for East of Eden, while giving the studio the option to extend his contract for several more films, which it did with Rebel Without a Cause and Giant. The auction house describes it as “one of the most important acting contracts in the history of Hollywood” (bidding begins at $3,000).

There are letters that give a rare glimpse inside Dean’s mind and of his writing style. He sent two handwritten letters to Deacy (bidding for each begins at $3,000). In the first, he addresses Deacy as “Mom”, writing about life in Los Angeles while filming East of Eden.

In the second, a two-page epistle written in blue fountain pen on a single sheet, he acknowledges personal issues such as family problems and “discreetly” looking for a dentist since he was sensitive about his false front teeth; he hand-draws a small self-portrait showing his “pigmented” tongue.

Dean writes: “Dear Jane / I had a wire sent to my bank (Chase) and their reply to my frantic question was $121.00. Sure went fast. I had to give up the rented car idea. It cost me $138.00 the two weeks that I had it. I hope that I still have the checks from Danger coming. I can’t live with my stepmother another minute and I’ll have to find a mode of transportation. As you know without a car your [sic] good as dead in Calif. I hate this place anyway. Please tell me that I still have the Danger checks because I love you.

“Have found a good dentist after a long and discreet search. Cliffton Webbs dentist (oh well). Have been going to a throat specialist because of a pigmented tongue [self-portrait drawing]. Too much smoking. Has stopped completely, also stopped drinking. Taking vitamins also. I was really run-down. I’m fatter now and feel much better. Tear myself up in New York. Build myself up in Calif. (health and maybe career huh?) The address at my fathers place is 1667 So. Bundy Drive, L.A. 19, Calif. or write to Dick Claytons office address and tell me that I have still the Danger checks. Returning home after picture to find analyst. (Is that spelled right, doesn’t look right). Miss you / Love / Jim.”

The collection also includes Dean’s motorcycle registration card from December 1953, shortly after he bought his Indian motorcycle. The card has Dean’s name and New York address printed five times, along with the motorcycle manufacturer, partial VIN and title number (bidding starts at $100).

Dean’s talent burned twice as bright and half as long and continues to fascinate long after his death. Sanders reflects: “He and Marilyn Monroe from that era seem to just have historic longevity. They’re always thought of when you think of classic Hollywood and they just never go out of style. And he died so young at 24.

“That age and his good looks and being great on the screen, it was just a winning combination and that’s why his autographs are collected 68 years later. He’s iconic. He’s one of the all-time great actors. He’s like Marlon Brando except Marlon Brando didn’t die aged 24. James Dean did.”

• Bidding via Nate D Sanders Auctions ends on Thursday and is expected to fetch tens of thousands of dollars.