tobacco:

One cannot underestimate the role tobacco and smoking plays in the Canon. Here are a few examples:

• A three-pipe problem is one so vexing Holmes must smoke three bowlfuls — REDH: “It is quite a three pipe problem, and I beg that you won’t speak to me for fifty minutes.”
• His love of strong tobacco — HOTB: “When you pass Bradley’s, would you ask him to send up a pound of the strongest shag tobacco?”)
• His monograph on cigarette ashes — SIGN: Upon the Distinction between the Ashes of the Various Tobaccoes — “In it I enumerate a hundred and forty forms of cigar-, cigarette-, and pipe-tobacco, with colored plates illustrating the difference in the ash.”
• The Persian slipper in which he keeps his tobacco
• His meerschaum pipe (note this is intentially misleading so please follow the link).