Learn Hardcore Korean: harue chaegeul han gwon ilgeoyo.

하루에 literally means in one day or per day. The particle 에, when attached to time nouns, indicates the time when or the time span in which an action occurs. Here it expresses the rate: one book per day.

Why is 책을 marked with 을 instead of 를?

을 is the object-marking particle used after nouns ending in a consonant. Since 책 ends in the consonant ㄱ, we use 을 to mark it as the direct object. If the noun ended in a vowel, we would use 를 instead.

Why does the sentence use 한 권 instead of 한 개 or just 한?

권 is the native Korean counter for books and similar volumes. Different counters are used for different object types. 개 is a general counter but is less natural for books; 권 is the correct choice here. So 한 권 equals one book.

Why is there no subject like I or we in the sentence?

Korean often omits the subject when it is clear from context. In this sentence, the speaker is talking about their own habit, so I or we is understood and does not need to be stated explicitly.

What tense and level of politeness is 읽어요, and how is it formed?

읽어요 is the present-tense polite form of the verb 읽다 (to read). You remove 다 from 읽다, leaving 읽-, and then add -어요, which is the standard polite ending. This makes it polite and present tense.

Can I reorder the words? For example, is 책을 하루에 한 권 읽어요 correct?

Yes, Korean word order is relatively flexible, but it generally follows the pattern: Time – Place – Object – Verb. Placing 하루에 before 책을 한 권 sounds more natural, but 책을 하루에 한 권 읽어요 is still understandable and grammatically correct.

Why isn’t 책 pluralized with 들 as in 책들?

When you use a counter like 한 권, the noun itself remains in its basic form without the plural marker 들. The counter already indicates the quantity, so you don’t need 책들 here.

Could I say 매일 한 권 읽어요 instead of 하루에 책을 한 권 읽어요?

Yes. 매일 한 권 읽어요 means essentially the same thing: I read one book every day. 하루에 emphasizes the rate per day, while 매일 simply means every day.