The question here is whether the antecedent for the relative pronoun which should be simply “the book” or “the words of the book”. The earliest text reads “which were”, which implies that the which refers to the head of the noun phrase (namely, words). The first printing of the 1852 edition replaced the plural were with the singular was, but in the second printing the original were was restored, probably by reference to the 1840 edition. Usage in the previous verse shows that both the words and the book can be sealed:
Thus we follow the earliest textual sources in determining whether the words are sealed or the book is sealed. For other examples, see under subject-verb agreement in volume 3.
Summary: Maintain the earliest reading in 2 Nephi 27:11: “the words of the book which were sealed”.