The stories vary widely, ranging from retellings of canonical Holmes tales from different perspectives, entirely new adventures featuring Holmes and Watson (including one in which he assists in the assassination of President McKinley), modern-day mysteries inspired by Holmes (or Holmesiana), and even a story from the perspective of Mrs. Hudson, Holmes' landlady at 221B Baker Street.
I particularly liked Alan Bradley's "You'd Better Go In Disguise," Tony Broadbent's "As to 'An Exact Knowledge of London'," Lee Child's "The Bone-headed League," Neil Gaiman's "The Case of Death and Honey," and Charles Todd's "The Case that Holmes Lost."
A great book to spend a nice autumn weekend with, I found. It'll remind you of all the best parts of the Holmes stories, and make you want to read and enjoy them all over again.