As a narrative device, the juxtaposition between the men at the different points in their lives, and London herself, help give the story a quick pace that clicks along well.
The friends have seven decades of work partnership and friendship behind them and although this is only the first installment of an entire series the depth of and intimacy of their lifetime together are portrayed convincingly. Not since Holmes and Watson has a deep, sincere and affectionate friendship between two men been better portrayed. Bryant and May have been the constant in each other's lives, through marriages, divorce, children, grandchildren, the introduction of technology in their work and a myriad of small and large markers of the passing of time.
Third, the dual mysteries are both well plotted, though I did figure the solution out long before the end of the book. That did not, however, keep me from wanting to continue reading.
Finally, how many series begin with the death of a main protagonist?