"Holmes noted it down and sat, still smiling with the open memorandum-book upon his knee."
-- "The Illustrious Client"
It really sounds like something important, this memorandum thing, but less so when one calls it by the modern short-form, the "memo." It's even less fancy seeming when one looks up the definition: a note or message used in business or diplomatic settings. So Sherlock Holmes has his business notebook on his knee. And every bit of glamour goes right out of the consulting detective business.
Because the business just became more business-like. Can you imagine if one of the collections had been titled "The Business of Sherlock Holmes?" Whew.
Who else uses these business memos in the Canon?
Well, Jonathan Small thinks Pappy Sholto might have written treasure memos, but no. Belgrade sends them to England so they can sit in Trelawney Hope's dispatch box. And Professor Moriarty carries one around with him . . . which makes it odd that Sherlock Holmes has never been seen with a memorandum book until after he sees that Professor Moriarty carries one.
But is it Holmes taking cues from his arch-nemesis, or is it just that, by 1902 and the memorandum-book, the "agency" he refers to in "Sussex Vampire" is an actual business grown beyond his one-man-plus-Watson-sometimes show? Or a worse theory -- was his memory starting to go by 1902, en route to the condition portrayed in the movie Mr. Holmes? (Though if things were already going downhill in 1902, I doubt brother Mycroft would have sent him to spy in American a decade later.)
My worst take on that memorandum-book is that it's the one clue that Moriarty was actually the one that survived Reichenbach and took Holmes's place, and "Empty House" was really a twisted take on Moran killing Watson then taking his place at the side of the new "Sherlock Holmes" as the new "Dr. Watson."
Surely they couldn't have gotten away with that, could they?
And here I thought memos and memo books were just dull, dull, dull. You never know what's around the corner with Sherlock Holmes.