Lovecraft never used a typewriter. He wrote all of his fiction and his 100,000 letters by hand.
He did own a typewriter -- a rebuilt Remington of 1906 -- but loathed typing. I've been told this was because his Remington was an under-strike model. But there are typescripts and even letters that he typed himself.
It was de Camp who came up with the number "100,000 letters", probably just by guessing. Joshi has since adjusted that number downwards to 60,000-80,000, but that's still a sizeable number.
Cool Air is one of my favorites by HPL and was a huge influence on a Lovecraftian story I wrote years ago called On A Winter's Eve. I enjoy trying to capture the eerie, dread-building that Lovecraft was so capable of, while at the same time crafting a 'hero' that is not quite so .... spineless. That's where I fallback on REH's influence, introducing Herculean strength and bravery to a character who is faced with something supernatural and dreadful. By the end of Winter's Eve, my character as well, however, can no longer stand a 'draught of cool air', albeit for entirely different reasons.
I'm currently reading a new edition of The Call of Cthulhu in the Penguins Classics Deluxe Edition, annotated by S.T. Joshi that is fantastic. Any annotated Lovecraft is a must-read for the true fan as you get so much more out of the text once you get the 'behind the scenes' scoop. The binding on this 'white' edition as I call it is neat, with rough cut edges reminiscent of a bygone day and the story selection is great.