Captain Bartlett was the commander of the Britannic at the time of her sinking. I was given the opportunity to purchase his signature which came from a nurse's autograph book. He signed her book just 2 weeks prior to the sinking.
If I'm not mistaken, this particular item has been on Ebay a while back. Can't remember exactly what happened to it, but it went for quite a price I thought. Someone with a better memory?
As to how rare it is, not much (read: almost none) items relating directly to Bartlett are offered on Ebay; certainly not items with a personal touch as an autograph. But on the other hand - with his long and active career many items will exist, but mostly in archives.
Thanks for your insights. Yes, you are correct as to the signature being offered on ebay last July. It did go for quite a hefty sum, but it failed to meet it's reserve price. Apparently after the auction ended, the seller sold it to a collector who bounced a check on him. The page was signed onboard by the captain. After the sinking this information was written on the page:
"H.M.H.S. "Britannic sunk by mine or torpedo, in Zea Channel off Athens Nov. 20, 1916, - fourteen days after the captain signed my book.