The book of Dubliners is written with style of scruplous meanness. How I do not understand.

Taken just as you've written it, without examining the
context, you'd need to center on "scrupulous meanness."  Joyce says that he wrote
Dubliners with scrupulous
meanness.

Scrupulous means to have in
mind what you believe to be right.  It also means exactness and precision.  So Joyce
says that he wrote what he thought was right, precisely and exactly.  He might also mean
that he arrived at what he thought was right by exact and precise
study.

Understanding what Joyce means becomes more
difficult, though, when one adds meanness to the thought.  He could
mean one of two things, or both.  First, he could mean that what he thought was right
led him to meanness.  Second, he could mean that he was not mean carelessly, that his
being mean was the result of what he thought was right. 

In
short, the stories are the products of what Joyce thinks is moral or right.  One might
say that they demonstrate Joyce's "righteous indignation," without the usual religious,
simplistic, and moralistic connotations those words might usually
suggest.

In Dubliners, the
shortcomings of Dublin residents, and the Irish as a whole and humans in general, are
harshly exposed.  Humans can be ignorant, superstitious, manipulative, psychologically
impotent and paralyzed, etc.  These weaknesses are exposed in the stories.  One could
conclude that the stories are written with "meanness."

Yet,
the stories are not didactic--preachy or sermon-like.  Joyce's narrators do not intrude
(and neither does he as author) to condemn Dublin residents.  Joyce does not appear
to directly condemn anyone.  Instead, weaknesses are revealed by
narrative. 

In short, Joyce exposes weaknesses, but, he
says, he does it based on what he believes to be right, and he does it with exactness
and precision.  He is mean, but his meanness is based on what he carefully considers to
be right.