Welches Buch wirst du lesen?
Why in the sentence "Welches Buch wirst du lesen?", it is used lesen instead of liest?
The present tense is "du liest", so your sentence in the present tense would read "Welches Buch liest du?". However, the sentence you quoted is written in the future tense. The future tense consists of two parts: 1. the inflected ("changed") form of the verb "werden" (to become) and 2. the infinitive of another verb. Only the verb "werden" is changed according to the person doing the action (here: du), while the infinitive of the other verb always remains the same. E.g. Ich WERDE lesen (I will read), du WIRST lesen (you will read), er WIRD lesen (he will read), and so on. It has nothing to do with the noun "book".
Because wirst is corresponding with the noun book, lesen is the infinitive. This is like in English, we say "which book will you read?" not "which book will you reads?", which is what would happen if we put "welches Buch wirst du liest?".