Death at the Bar, by Ngaio Marsh

Like Overture to Death, this murder mystery is set in a small
village–in this case, a seaside village in Devon. And also like
Overture to Death, it has a very long lead-in before the
murder is even committed. But instead of being about the tensions that
naturally arise in a small village, it’s about the tension between the
locals and visitors: in this case, three friends–a barrister, an actor,
and a painter–who have come to stay at the local inn for a few weeks for
the second year in a row.

One of them is killed in the inn’s common room, and Alleyn (helped, most
unusually, solely by the stalwart Inspector Fox) must determine not only
who killed him, but also how. It’s rather like a locked-room mystery–we
know he was poisoned, but except for the evidence of poison in his blood,
it seems impossible that he could have been.

I didn’t like this one much when I first read it–other than Alleyn and
Fox I found few of the characters to be particularly sympathetic–but it
began to grow on me this time.