Explain why Collins does not complain when Lydia interrupts his reading of Fordyce's "Sermons," but criticizes her in his letter in Ch.57.

Collins arrives at Mr. Bennet's house in Ch.13. In the
next chapter after he has had his dinner he is seen in conversation with all the members
of the Benett family. Its very obvious that he is doing his best to make a very good
impression and win the admiration especially of the Bennet girls, because he has come to
Longbourn with the main intention of marrying one of
therm.

Soon after dinner and tea the family moves to the
drawing room where he tries to impress the Bennet girls  by reading aloud to them. To
the shock and surprise of both Kitty and Lydia he announces that "he never read novels"
and begins to read from Fordyce's "Sermons" - an obviously dull, sentimental and
moralistic book - only to be rudely interrupted by
Lydia.

However, Collins who
is

"much
offended, laid aside his book, and said,

I
have often observed how little young ladies are interested by books of a serious stamp,
though written solely for their benefit. It amazes me, I confess; -- for certainly,
there can be nothing so advantageous to them as instruction. But I will no longer
importune my young
cousin.''

He is very
courteous and polite because he doesn't want to offend the girls by being very
moralistic otherwise none of them would agree to marry
him.

However, after he has married Charlotte he reveals his
true colors. Collins is a hypocrite and he pours out his venom in his letter in Ch.57.
He does so in order to avenge his shame and humiliation in being rejected by Elizabeth
who is now on the verge of marrying Darcy and by being snubbed by Lydia in Ch.
14:

readability="11.692189892802">

I am truly rejoiced that my
cousin Lydia's sad business has been so well hushed up, and am only concerned that their
living together before the marriage took place should be so generally known. I must not,
however, neglect the duties of my station, or refrain from declaring my amazement at
hearing that you received the young couple into your house as soon as they were married.
It was an encouragement of vice; and had I been the rector of Longbourn, I should very
strenuously have opposed it. You ought certainly to
forgive them as a Christian, but never to admit them in your sight, or allow their names
to be mentioned in your
hearing.''