OK, I promised I would not analyse this episode. My partner had other ideas- and I watched it again. I caught this:



That is Katherine Neville's The Eight, from 1988 on the floor at the very beginning of the 221B cleanup montage. It is in fact, the very first shot we see in this sequence- and placed very deliberately. Sherlock's hand appears and picks up the magnifying glass, which is right behind the book. 

What is significant about "The Eight"? Well, it about the quest for a legendary chess set, through different timestreams. The story is full of codes and encypted messages. 

"What makes The Eight an extraordinary novel, then, is its capacity to combine fiction with documented historical elements, and fit them into a narrative full of connections between such diverse subjects as music, art, history, mythology, physics, chemistry, mathematics, philosophy and, in more speculative fields, mysticism, metaphysics and alchemy, among other esoteric traditions. Despite telling the story of the imaginary search for an imaginary object by a group of imaginary characters and, despite the fact that its narrative is centered on an element of fantastic properties, the reading of The Eight oozes narrative realism, especially in its first two thirds. The reason for that, aside from these historical characters and this chess-related framework, which we’ll soon analyze, is the the careful spatial and temporal composition of the novel."

http://www.katherineneville.com/about-my-work/my-bookessays/1581-2/

Also, Chapter 13 features a Sherlock Holmes quote: “I play the game for the game’s own sake”. 

Clearly, its placement is intentional, meant to be noticed, but what do you make of it? 'Poetry, or truth'?