...and grazing over information that is important in Deathly Hallows.
I suppose that the very basic plot is the same, [horcruxes, dead Dumbldore, yeah, yeah] but this new movie seems very loosely based on the book.
I enjoyed the Half Blood Prince a lot and would like to start reading the final book (I did not read any of the others) so I don't have to wait till the last movie to see the ending.
I'd be very grateful if you could go into detail about how the book was different in the places you mentioned and what I should know before reading Deathly Hallows.
I'd also like to know why Snape calls himself the "Half Blood Prince" unless of course it is in the last book. I'll just read that one myself.
Indja is right about the "Half Blood Prince" thing.
Okay, I'm going to go through the chapters and tell you things that are important for reading Deathly Hallows.
[or just things that they omitted and aren't necessarily important but come up]
Chapter 1: The Other Minister
-Cornelius Fudge is no longer Minister of Magic.
-Rufus Scrimgeour is the new Minister.
Chapter 3: Will and Won't
-Harry inherited Number 12 Grimmauld Place, Kreacher the house elf and Buckbeak the hippogriff from Sirius.
-Dumbledore informs the Dursleys that Harry must return to their home one more time after this summer. The reason for this is that Dumbledore put protection on the house so that Harry cannot be harmed within it. This ends when he is 17.
Chapter 5: An Excess of Phlegm
-Arthur Weasley has been promoted as the head for the Office for Detection and Confiscation of Counterfeit Defensive Spells and Protective Objects.
-Fleur Delacour [from Goblet of Fire] is engaged to Bill Weasley.
Chapter 6: Draco's Detour
-The wandmaker Ollivander has been kidnapped.
-In the movie, Fred and George's joke shop is introduced. The are able to pay for this because Harry gave them his Triwizard Tournament winnings of 10,000 Galleons in Goblet of Fire.
Chapter 10: The House of Gaunt
-This is Dumbledore's first journey into the Pensieve. The follow Bob Ogden, a Ministry representative to the house of Marvolo Gaunt. He lives there with his two children, Morfin and Merope. You find out that they are descendants of Salazar Slytherin, the founder of Slytherin House. Morfin is in trouble for torturing a muggle by the name of Tom Riddle. He then goes on to say that Merope fancies Tom Riddle. A fight ensues with Bob Ogden, and Marvolo and Morfin are taken to Azkaban. In their absence, Merope brews a love potion and gives it to Tom Riddle. They stay together for long enough that Tom Riddle impregnates Merope, but she stops giving him the love potion and he leaves. Merope dies a little while after giving birth to the baby. Merope is Voldemort's mother, and the father is the muggle Tom Riddle. Therefore, Voldemort is a descendant of Salazar Slytherin.
Chapter 12: Silver and Opals
-This seems relatively innocuous, but Mundungus Fletcher, a member of the Order of the Phoenix is stealing Sirius' possessions from Grimmauld Place.
Chapter 13: The Secret Riddle
-In another memory in the Pensieve, Merope takes a family heirloom [Slytherin's locket], and sells it to Borgin and Burkes.
-In the movie, you saw Dumbledore's memory of visiting Tom Riddle[young Voldemort] in the orphanage where he grew up. One thing not included is when Dumbledore finds a box of Tom Riddle's that contains possessions of other children. He collected these things as trophies. For example, if he harmed a student, he might take their toy as a trophy. Dumbledore showed Harry this memory to explain that Voldemort always takes trophies.
Chapter 16: A Frosty Christmas
-Also rather innocuous, but you find out that Lupin was bitten by Fenrir Greyback when he was young, and that's how he became a werewolf.
-There is a part in the book where Rufus Scrimgeour and Percy Weasley come to the house. What happened is unimportant, but the mention of Percy reminded me of something they left out in Order of the Phoenix - the reason why Percy has been absent from the films. In Order of the Phoenix Percy is promoted to a position where he works closely with the Minister. Arthur urged him to abandon this and stay with the Order of the Phoenix. Percy and his father had an argument. Percy told his father that he was holding the family back and was giving him a terrible reputation at the Ministry. This led to Percy moving out severing all ties with his family.
Chapter 17: A Sluggish Memory
-Dumbledore takes Harry into the Pensieve where they are looking at Morfin Gaunt's memory. He has been released from Azkaban by this time, but his father is dead. Young Tom Riddle [Voldemort] comes to his home and kills him, stealing Marvolo Gaunt's ring. [which was once Slytherin's] After he did this, he went over to his muggle grandparents house and killed them, as well as his father. He then Confunded Morfin Gaunt to make him believe that he had killed them. Morfin went to Azkaban for this and died there.
Chapter 20: Lord Voldemort's Request
-Dumbledore and Harry go into the Pensieve into a house elf's memory. Voldemort's first job out of school was at Borgin and Burkes. He was a very persuasive man, so he was often sent to collect valuable items from people. He goes to a woman's home, Hepzibah Smith, where she shows him two things that she owns: Helga Hufflepuff's goblet and the locket of Salazar Slytherin. She bought it from Borgin and Burkes shortly after Merope sold it. Voldemort was highly interested in items that belonged to the founders of Hogwarts. A few days after this visit, Hepzibah Smith was found dead, the goblet and locket missing.
-In another memory, Voldemort applies for the Defense Against the Dark Arts job.
Chapter 23: Horcruxes
-I forgot if this was in the movie, but you figure out that 2 of the 7 Horcruxes have been destroyed already: the ring and the diary. Dumbledore speculates that the remaining ones are Helga Hufflepuff's goblet, Salazar Slytherin's locket, something of either Gryffindor's or Ravenclaw's and Voldemort's snake, Nagini. Only when all of these are gone can Harry kill Voldemort.
Chapter 24: The Seer Overheard
-It comes to light that Snape overheard Trelawney making the prophecy. Snape told Voldemort about the prophecy, which led him to go to the Potter's home and kill them.
Chapter 27: The Lightning-Struck Tower
-This is in the movie, but Draco goes up to the astronomy tower and disarms Dumbledore just before Snape kills him. This may not seem important, but I'm stressing it because it is a crucial detail in the end of Deathly Hallows.
Chapter 29: The Phoenix Lament
-There was a fight at Hogwarts between the Death Eaters and the school. The Order of the Phoenix was there. Fenrir Greyback was with the Death Eaters, and he attacked Bill, severely scarring his face. Bill is not a werewolf, because Greyback was human when he attacked.
Chapter 30: The White Tomb
-Harry sort of breaks up with Ginny because he knows that he must go on a journey to find Horcruxes, and he is afraid of people targeting Ginny.
Okay, I think I've pretty much got it.
If you start reading the book and you don't understand a certain detail, I think I should be able to explain.