"You have never treated Harry as a son. He has known nothing but neglect and often cruelty at your hands. The best that can be said is that he has at least escaped the appalling damage you have inflicted upon the unfortunate boy sitting between you."
-- Albus Dumbledore to Vernon and Petunia Dursley
He comes to collect Harry but needs to settle a few details first.
Thinking Dumbledore coming to take him on an errand and then to the Burrow might just be too good to be true, Harry hasn’t packed or told the Dursleys about this when Dumbledore arrives on Privet Drive (an hour late) at midnight. Using confidence and magic to intimidate the Dursleys into playing host reluctantly, Dumbledore informs Harry that Sirius left his godson everything in his will (and thereby letting the Dursleys know Sirius is dead). Dumbledore also conjures up some mead, and pours it into glasses that then persistently knock against the Dursleys’ heads. Along with Sirius’ gold, Harry inherits Number 12 Grimmauld Place. However, the Order of the Phoenix is concerned that some magic may have been placed on the house to prevent it from falling into the ownership of anyone other than a pure-blood member of the Black family. To test this, Dumbledore summons Kreacher to the Dursleys’ living room, much to their dismay. The existence of of an such magic is disproved by the resentful Kreacher’s inability to disobey an order from Harry, who sends the house-elf to work in the kitchens at Hogwarts. Harry also decides that Buckbeak (now going by the alias “Witherwings”) can stay with Hagrid. Dumbledore then informs the Dursleys that, for the magic on their house protecting Harry to continue to work until Harry comes of age at 17, they must let him return to Number 4 Privet Drive one more time. Dumbledore then admonishes Vernon and Petunia for their treatment of Harry and of Dudley before taking Harry away.
In his letter to Harry, Dumbledore says he will call at 11:00 PM on Friday. Since he is one hour late, he arrives just at 12:00 AM on Saturday.
Dumbledore appears to use the deluminator to put out the street lamps when he arrives on Privet Drive. This seems odd since he goes right inside the Dursleys' house. And there is not mention of him returning light to the street before he and Harry disapparate (HBP4).
Dumbledore remarks that the Dursleys' "agapanthus are flourishing" when he states that has not visited Number 4 Privet Drive in a long time. This may be read as an observation that Lily Potter's son son is growing into a fine young man since agapanthus is the botanical name for the flower more commonly known as the African lily (source: Royal Horticultural Society). -BB
Harry wants nothing to do with Kreacher, but Dumbledore points out that it is a matter of responsibility for Harry since Kreacher knows all the about the work of the Order of the Phoenix and could tell their enemies if he were freed. This is similar to the situation that led Hermione to keep Rita Skeeter (in beetle form) trapped in a jar so she could not share delicate secrets with her readers (GF37). -BB
According to the tradition of inheritance observed by the House of Black the inheritor of Number 12 Grimmauld Place ought to be Bellatrix Lestrange, and it is clear that Kreacher wants the house to go to Sirius' murderer just as strongly as Harry does not want it to go to her.
There must be something magically binding as well as legally binding about wizarding wills since it automatically transfers a house-elf's forced loyalty to the beneficiary. -BB
Judging by his uncle's greedy interest in Harry's inheritance, Uncle Vernon's response of "preposterous" upon hearing that wizards come of age at age 17, is likely an expression of disbelief or frustration at learning that he has less time than he thought to try to take advantage of his legal dependent's inheritance before Harry can do as he pleases with it without any chance of interference from his aunt and uncle. Although just about anything and everything about Dumbledore and his visit could elicit a response of "preposterous" from Vernon Dursley. -BB
Dumbledore puts an emphasis on manners and uses them to irritate people who are hostile to him. This is in the third chapter in the book. Dumbledore does the same thing when he is cornered atop the Astronomy Tower in the third-from-last chapter of the book. -BB