This letter was met with "disbelief" when Christie's experts first saw it: a 1960 handwritten letter by Paul McCartney inviting an unknown drummer to audition for The Beatles.

The letter, dated August 12 of that year, was discovered folded-up inside a book by an anonymous collector at a car boot sale in Bootle, Liverpool, UK.

Prior to the sale, Neil Roberts, Director of Popular Culture, Christie's, described the letter as "something so extraordinary, it alters the knowledge of your specialist field."

"My initial reaction was one of disbelief," he said.

"But on seeing the item and being able to research the significance of the date and its content as well as conferring with renowned Beatles historians, it has turned out to be much more significant than mere words on paper."

Collectors apparently agreed when the piece appeared for sale in London, earlier this week (November 15). Estimated at just £7,000-9,000, the note eventually sold for £34,850.

The unique letter appeared among more than 30 lots dedicated to The Beatles and its individual band members.

Other highlights in the sale include an iconic authentic "Bed Peace" artwork by John Lennon. It brought £97,250 ($154,000), was the only constant protest sign used during Lennon and wife Yoko Ono's famous "bed-in for peace" in a Montreal hotel in 1969.

Fab Four collectible sales are continuing to bring greater values at the world's top auction houses. Autographs are especially valued.

For instance, if you'd owned a Beatles autograph signed by all four members 11 years ago worth £2,950, it could today be worth as much as £8,9590 - that's a rise in value of 203.4%.

Key to the growing values of Beatles memorabilia is their undying legacy and influence in music. Lennon is currently placed fifth on Forbes' 'Top Earning Dead Celebrities' list with posthumous earnings of $12m, largely thanks to publishing revenue from his classic songs.

And McCartney hasn't been left out. Forbes recently listed him as the 20^th most powerful person in entertainment earlier this year.

This group autograph for fan “Helen” by John, Paul, George – a cheeky forgery of “Ringo” perhaps written by Paul – is currently for sale at Paul Fraser Collectibles – we have priced it at just £9,950 with free insured delivery

That McCartney's letter to an unknown drummer nearly tripled its £7,000-9,000 estimate earlier this year will be partly down to competition among bidders and the adrenaline rush of a live auction.

But, the good news is, if you're a collector on the lookout for an historic Beatles collectibles there are opportunities available on the private markets - for a fraction of the price.

Like this rare and authentic example featuring Beatles Paul, John and George's autographs on message to a fan named "Helen". (Ringo's name also appears, although we believe it wasn't penned in his own hand but possibly by McCartney.)

It's currently for sale at Paul Fraser Collectibles priced at £9,950 with free insured delivery.

And, given the 203.4% rise in value that The Beatles' autographs have enjoyed over the last 11 years, this piece will surely bring a lucky collector passion, pleasure and profit in future years.

Watch this space for all the latest Beatles collectibles news.