"My son... my only son..."
"You should be proud!" said Bellatrix ruthlessly. "If I had sons, I would be glad to give them up to the service to the Dark Lord!"
Narcissa gave a little scream of despair and clutched at her long blonde hair. Snape stooped, seized her by the arms, lifted her up, and steered her back onto the sofa. He then poured her more wine and forced the glass into her hand.
"Narcissa, that's enough. Drink this. Listen to me."
She quieted a little; slopping wine down herself, she took a shaky sip.
"It might be possible... for me to help Draco."
She sat up, her face paper-white, her eyes huge.
"Severus--oh, Severus--you would help him? Would you look after him, see he comes to no harm?"
"I can try."
She flung away her glass; it skidded across the table as she slid off the sofa into a kneeling position at Snape's feet, seized his hand in both of hers, and pressed her lips to it.
"If you are to protect him... Severus, will you swear to it? Will you make the Unbreakable Vow?"
"The Unbreakable Vow?"
Snape's expression was blank, unreadable. Bellatrix, however, let out a cackle of triumphant laughter.
~The Half-Blood Prince, Ch. 2 "Spinner's End"
I cut off there because I didn't want to go through all of the Unbreakable Vow. This is one of my favorite chapters in the whole series, mainly because we finally get to see Snape's house. I would seriously love to live at Spinner's End... I mean, I don't care about the threadbare furniture- in fact, that's even better! But I'd love to have the walls as bookcases and the entrances to all other rooms hidden in them. Plus, Snape is there! I bet he gave his old childhood bedroom to Wormtail and now sleeps in the master bedroom that his parents used to have... Well, it had bad memories anyway, so after Wormtail leaves he probably won't even want to go in there. He and I can share a bed. ;D
Anyway, another reason I love this chapter is that we actually get to see what Snape is doing even though Harry doesn't. It's a chapter that's basically allowing us to choose whether or not we trust Snape. Before we read Deathly Hallows, of course. But since we see what he's doing but not inside his mind, we're not sure of his motives. We're also not sure what exactly Draco has to do, but we know it's something nefarious and dangerous. Those who thought Snape was on Voldemort's side were screaming at Harry throughout the whole book, "WATCH OUT FOR SNAPE OMG." And those who trusted him were pretty surprised when he ended up killing Dumbledore.
Another thing about this chapter is that it's hilarious. When Bellatrix said that she'd happily give up an sons she had to the Dark Lord, I'm a little creeped out and am thinking 'Well that's why you're not allowed to have children, Bella.'
And then, all the fangirls squee'd at this part- Narcissa kissed Snape's hand. First of all, I'm wondering what Snape was thinking when it happened. He probably hadn't had a woman kiss him anywhere for like... 20 years. It also proves to everyone who doubted us that Snape is desirable. If he was so ugly, then wouldn't Narcissa have been repulsed at the idea of kissing even his hand? People usually argue that he's described as ugly for much of the books, but that's just what Harry and Ron think. They're guys, and they hate him, so obviously they're not going to say he's attractive. Some people might find him amazingly sexy.