Continuing someone else's series: Randall Garrett and Michael Kurland

Randall Garrett, who wrote an immense amount of science fiction/fantasy (17 novels and hundreds of short stories), also wrote a (brief) alternative-universe series of mysteries.  In this series, Richard the Lionhearted did not die, his brother John did not become king (and presumably, also, the Magna Carta was never written), and the Plantagenet dynasty continued into the 20th century.  The series won the Sidewise Award for Alternate History Special Achievement Award in 1999 (initiated in 1995, and usually awarded at the SFF World Con) Garrett's three books were

Too Many Magicians (1966; novel)
Murder and Magic (1979; shorts)
Lord Darcy Investigates (1981; shorts)

Garrett died in 1987, just days after his 60th birthday.

(All that information comes from Wikipedia articles; google Garrett by name and the name of the award if you want more information.)

Michael Kurland (SYKM page: http://www.stopyourekillingme.com/K_Authors/Kurland_Michael.html), who, in his introductions to the two books he wrote, described Garrett as a good friend, took up the series after Garrett's death, publishing two Lord Darcy novels, Ten Little Magicians (1988) and A Study in Sorcery (1989); I don't need to tell you the sources for those titles.  Both are available  now as ebooks, which is how I found them. 

Ten Little Magicians involves the investiture of King John's heir to the throne, which is threatened by a series of killings of wizards.  Each of the killings is accompanied by a poem based on the same nursery rhyme Agatha Christie used.  Here's the first one:
"Ten little wizards sat down to dine.
One wizard stuffed his face--and now there are nine"

It's well-written, and moves nicely, but the killer is fairly obvious, as is how he committed at least one of the locked-room mysteries in the book.

In A Study in Sorcery (1989), Darcy and his magician associate are sent to "New England"--North America as a whole--involving the strange death of an Aztec prince just prior to the planned signing of an Anglo-Aztec treaty.  Generally well-done, but the murderer is (again) fairly obvious.

Kurland does a nice job of continuing Garrett's creation of an essentially medieval society in the 20th century, lacking much of the technology we take for granted, and in which magic works.  In fact, in some ways his plots are stronger than Garrett's,  As a hook for a series, it's interesting, but I suspect that it would have become difficult to continue for much longer, even had the books sold well enough.

Kurland has written a lot, but none of his series characters seems to generate enough of a following to be continued.  His Moriarity series seems to have done better than the others {with 5 entries spanning 1979 [the Edgar nominated The Infernal Device (for best paperback original) to 2014 [Who Thinks Evil]}.

I've found Kurland's books to be readable and fun, but not overly compelling; I think the two Alexander Brass books (Too Soon Dead and The Girls in the High-Heeled Shoes) are by far his best work.