A romp through a magical alt-present (1966) London where the Plantagenets still rule and magicians take the place of scientists and engineers. There are multiple murders and a various red herrings before the fish is netted. ( )

Sherlock Holmes with magic. Fun read. ( )

dreadful cover. Nothing to do with the book at all! The book is great. ( )

Substance: Lord Darcy solves another baffling case through ratiocination, one-upping his cousin the Marquis of London, and without much of the usual aid from his side-kick Master Magician Sean O Locklainn.
The clues are fairly laid and, as with all the Darcy stories, the deductions are not dependent on magic any more (or less) than Holmes' are on forensic analysis: Master Sean's evidence is only one strand among others. The conclusion's satisfactory twist owed part of its coil to the existence of magic nonetheless. It also owes a great deal to Christie's "The Murder of Roger Ackroyd" IYKWIMAITYD.
I am personally very fond of the alternate-history world Garrett has created by continuing the Plantagent / Aquitaine dynasty in an Anglo-French empire; the introduction of magic as a major construct (very civilized, not sword-and-sorcery) is a bonus.
FWIW, Darcy's given names are never given. He is the Lord of Arcy in France (Lord d'Arcy).

Style: Garrett's style here is reminiscent of the punctilious use of titles and courtly etiquette typical of Heyer's Regency romances, although the sole romantic entanglement is only a lightly touched element. One of the pleasures of the Darcy stories, abundant in this, the only novel of the oevre, lies in spotting the puns, hat-tips, and parodies. Many "English" names are spelled in a French fashion, for one thing, and others are simple translations of well-known fictional figures. He also uses a large number of obsolete and archaic spellings and synonyms for today's vocabulary.

Nero Wolfe and Archie Goodwin are prominent role-models, down to cloning most of the essential elements of Stout's novels, including the house, orchids, and side-kicks. I spotted the analog to Gandalf the Grey, but one reviewer claims to have noticed James Bond as well, who I missed somehow.

NOTES:
NOTE: I have read the book before, and actually re-read the damaged copy that I trashed, rather than this particular one. ( )

Twenty years ago, I would have rated this book higher but it seems slow, overly complicated and dated. ( )