Chapter 23 contains not Mr. Collins' next letter but an abstract of it.

"The promised letter of thanks from Mr. Collins arrived on Tuesday, addressed to their father, and written with all the solemnity of gratitude which a twelvemonth's abode in the family might have prompted." No doubt it contained many effusions of appreciation for their bounty, hospitality & condescension which we have been mercifully spared.
"After discharging his conscience on that head, he proceeded to inform them, with many rapturous expressions, of his happiness in having obtained the affection of their amiable neighbor, Miss Lucas," Again, more rubbing of the Bennet's noses in their loss. "and then explained that it was merely with the view of enjoying her society that he had been so ready to close with their kind wish of seeing him again at Longbourn," adding insult to injury--at least for Mrs. Bennet, "whither he hoped to be able to return on Monday fortnight; for Lady Catherine, he added, so heartily approved his marriage that she wished it to take place as soon as possible, which he trusted would be an unanswerable argument with his amiable Charlotte to name an early day for making him the happiest of men." I am assuming that Charlotte will now be faced with a fair supply of unanswerable arguments for any whim of LC. I don't doubt that Mr. Collins will be the happiest of men, as happiness is not dependent on intellectual power.