In "The Landlady," how does Roald Dahl manage to hold the reader's interest throughout the story?

After a certain point in the story Roald Dahl holds the
reader's interest with foreshadowing. But in the early part he holds the reader's
interest just by describing a scene of perfect simplicity and and tranquility. What
could be more safe and innocent than a homey  bed-and-breakfast establishment in a
stodgy town like Bath? The landlady is a typical sweet little old lady who keeps
everything neat and tidy and likes to chat with her
guests.

The reader, of course, senses that there must be
something sinister about such a place and such a landlady. We are suspicious just
because she seems so completely innocent. And this isn't only because we know what to
expect from Roald Dahl; it would be the same if any other author had written the story.
It is not unusual for a horror story to start off with a sentence such as: "It was a
beautiful, sunny summer day and the birds were singing in the trees." Here, for example,
is the opening of Shirley Jackson's shocking horror story "The
Lottery":

The
morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day;
the flowers were blossoming profusely and the grass was richly
green.

Here, by comparison,
is the opening sentence of "The Landlady":

readability="11">

Billy Weaver had traveled down from London on
the slow afternoon train, with a change at Reading on the way, and by the time he got to
Bath, it was about nine o'clock in the evening, and the moon was coming up out of a
clear stary sky over the houses opposite the station
entrance.

This absence of
foreshadowing is a kind of foreshadowing. We know something has got to
happen.

After Billy signs the guest-book, the
real foreshadowing begins. He notices the names of the two former guests and thinks he
has heard them but can't remember in what connection. At first the reader is not
suspicious of the tea the landlady is serving--but then it becomes obvious that she is
already involved in poisoning him.

readability="6">

The tea tasted faintly of bitter almonds, and he
didn't much care for it.

But
he has already consumed one whole cup, and his fate is sealed. It gradually dawns on the
reader--though not on Billy--that this crazy woman has become an accomplished
taxidermist. Billy realizes that the caged parrot is a stuffed parrot and then realizes
that the little dachshund he thought was sleeping so peacefully in his basket is a
stuffed dachshund. Still, it doesn't occur to him that this woman might have stuffed
Christopher Mulholland and Gregory W. Temple, even when she tells
him:

"Mr.
Temple, of course, was a little older. . . . He was actually twenty-eight. And yet I
never would have guessed it if he hadn't told me, never in my whole life. There wasn't a
blemish on his body."

"A what?" Billy
said.

"His skin was just like a
baby's."

How on
earth would she know that? Dahl doesn't describe Billy's reaction. The author
has specified, however, that his hero is very young, only seventeen years old. An older
man might decide to leave this place abruptly, because the most innocent thing this
landlady could have in mind is climbing into her guest's bed in the middle of the
night.

All the talk about Mulholland and Temple make the
reader strongly suspect that these men both received a lot of media attention because
they had mysteriously disappeared. We know what is going to happen to Billy when he asks
if there haven't been any other guests except those two men in the last two or three
years and the story ends abruptly with:

readability="7">

"No, my dear," she said. "Only
you."