I got a book from Judy which was more interesting than I expected.

[keannu]

My grammar book says only 2 is possible, but wouldn't 1 be possible with further context?

1. I got a book from Judy which was more interesting than I expected.
2. I got a book from Judy, which was more interesting than I expected

I got three books from my friends. I got a book from Judy which was more interesting than I expected, but the other two books from other friends were boring.

[bhaisahab]

My grammar book says only 2 is possible, but wouldn't 1 be possible with further context?

1. I got a book from Judy which was more interesting than I expected.
2. I got a book from Judy, which was more interesting than I expected

I got three books from my friends. I got a book from Judy which was more interesting than I expected, but the other two books from other friends were boring.

On it's own #1 is not possible. Your grammar book is correct.

[keannu]

Can you tell me why #1 is not possible?

[bhaisahab]

Can you tell me why #1 is not possible?

Because we can't tell whether it was the book, the receiving of the book, or (less likely), Judy, that was more interesting than you expected.

[keannu]

Is it not because of "a" in 1? #2 also has such ambuiguity for "which", I guess. The more I think about this, the more complicated it gets!

[5jj]

Is it not because of "a" in 1? #2 also has such ambuiguity for "which", I guess. The more I think about this, the more complicated it gets!

Perhaps you think too much, keannu. We had dozens of questions from you about possible interpretations of would, and you now seem to be embarking on a similar journey with commas and relative clauses.

Most of us native speakers do not consciously think about what we say. If we write, unless we hope to be published, most of us don't worry too much about the finer points of punctuation. Even if we hope to be published, we rely on our publishers and their editors to clean up our blunders.

Most of us, most of the time, assume that our listeners/readers are trying to receive a message from us, not trying to discover alternative interpretions of the words we have uttered/written.