“The Sign of Four” is the second Sherlock Holmes novella (“A Study in Scarlet” being the first). If “Scarlet” is readily available, I would recommend that one first as it spends time on introducing Watson and Holmes to each other. Actually, Sir Arthur Conan Doyle, the creator of Holmes, was not going to write another Holmes novella after “Scarlet,” but he was pushed into writing “The Sign of Four” after his first novella was so popular. “The Sign of Four” is an excellent lesson on deduction as a process of solving crimes and also introduces us to the British imperial forces located in their colony in Afghanistan. It also introduces us to Holmes’ drug addiction as he claims he was using it to combat boredom when he was in between cases.