Step 1: Recall the punctuation rules for direct speech.
- Quotation marks must enclose the spoken words.
- Commas are placed before the reporting clause ("I said").
- The sentence should maintain parallelism and logical pauses.
Step 2: Analyze each option.
- (A) Incorrect: Misplaces the quotation marks, splitting the sentence unnaturally.
- (B) Correct: Properly punctuated. Quotation marks enclose the speech, commas are correctly placed, and the sentence flows logically.
- (C) Incorrect: Misuses commas within the second quoted part ("but, a real book"). Also, closing punctuation should be inside quotation marks.
- (D) Incorrect: Omits quotation marks in the second part of the spoken words, making it grammatically incomplete.
Step 3: Conclusion.
Therefore, the correctly punctuated sentence is option (B):
\[
\boxed{\text{'One day I'll write a book', I said, 'not just a thriller, but a real book, about real people.'}}
\]