Why taran read the book of three?

It's impossible to answer that question definitively without more context. "Taran" could refer to many characters in different fictional universes, and "the book of three" could be a title of various books.

To answer your question, I need more information. Please specify:

* Which Taran are you referring to? Is it Taran from the Prydain Chronicles by Lloyd Alexander, a character from another fantasy series, or a real person?

* What book of three are you referring to? Is it a specific book title, or a general term?

Once you provide these details, I can provide a more accurate and helpful answer.