Since "lees" is the 2nd person singular of the verb "to read" (leer). "You read" is "tú lees". So that is why "you" is in the sentence. (It was omitted because it is implied in Spanish.)
If I wanted to say "Why read that book?", I think the translation would go like: "¿Por qué leer ese libro?". I'm drawing conclusions off of my French background. Can someone else confirm?
Jocelyn, when you know the gender of the noun, you use the demonstrative adjective that goes with that noun. ese libro, esa mujer. Eso is a demonstrative pronoun, not a noun, and indicates things or ideas that do not have a noun expressed for them, so we cannot know the gender. I like that. That is crazy. Me gusta eso. Eso es una locura.
“Why would you read that book?” would be in the conditional: ‘¿Por qué leerías ese libro?’.
“Why do you read that book?” is actually an odd translation for ‘¿Por qué lees ese libro?’, as the use of English present indicative with an action verb implies that the listener keeps reading it over and over. A much more plausible translation is “Why are you reading that book?”.
Nothing speaking against it. But it may be little unnatural in this situation, I'd say.
When you're asking that question, the book would usually be in the hands or within reach of the person you're talking to, which would warrant using ese. Aquel is used if it's out of reach for both of you.
It's a little awkward (and would sound a lot better with "for" placed at the end, for my taste).
But there's another reason why the translation might not be the most fitting. "Por qué" is asking for a reason: what drives you to read that book? "What for" rather sound like you're asking for the purpose: what do you want to achieve with the book-reading? If you're talking about a purpose, you should use "para qué" instead.
- ¿Por qué...? - Because of what...?
- ¿Para qué...? - To what end...?