The analyses of a text #5

The narrator recalls his adolescence.
He is an apprentice of the proctor, Mr. Spenlow.
He visited Mr. Spenlow's place and was introuduced to his daughter Dora and her confidential freind.
Miss muderstion is the narrator's ill-tempered step-father's elder sister.

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She was more than human to me. She was a Fairy, a Sylph, I don't know what she was - anything that no one ever saw, and everything that everybody ever wanted. I was swallowed up in an abyss of love in an instant. There was no pausing on the brink; no looking down, or looking back; I was gone, headlong, before I had sense to say a word to her.

'I,' observed a well-remembered voice, when I had bowed and murmured something, 'have seen Mr. Copperfield before.'

The speaker was not Dora. No; the confidential friend, Miss Murdstone!

I don't think I was much astonished. To the best of my judgement, no capacity of astonishment was left in me. There was nothing worth mentioning in the material world, but Dora Spenlow, to be astonished about. I said, 'How do you do, Miss Murdstone? I hope you are well.' She answered, 'Very well.' I said, 'How is Mr. Murdstone?' She replied, 'My brother is robust, I am obliged to you.'
[David Copperfield by Charles Dickens]
1. I think the subject of "to be astonished about" is I, but the subject of the main clause is "nothing."
So I was wondering it is grammatical.
2. I'd like to know what the object of "about" is.
3. I think "to" means "as to" in "I am not so foolish as to do so."
I'd like to know what "means "to be astonished about."
Thank you in advance for your help.