Learn Spanish (Latin America): El lector analiza el texto literario, lo que le interesa mucho. - The reader analyzes the literary text, which interests him a lot.

If we used just que, it would refer to the last noun mentioned, which is el texto literario. By using the neuter pronoun lo que, it refers back to the entire previous clause: the act of analyzing the text. It tells us that the analyzing is what interests him, not just the book itself.

Could I use lo cual instead of lo que in this sentence?

Yes, you can. When referring back to a whole idea or situation that was just mentioned in the same sentence, lo que and lo cual are generally interchangeable. Lo cual can sound slightly more formal or written, which fits well with a sentence about literary analysis.

If I split this into two sentences, could I start the second sentence with lo cual?

No. If you split this into two sentences (like 'The reader analyzes the text. This interests him a lot.'), you have to use lo que or eso to start the new sentence. Lo cual can only refer back to an idea connected within the exact same sentence.

Why does the sentence use le interesa instead of lo interesa?