"This book is really interesting."
"Books are people too!" Two reasons:
1) The pronoun ghaH is only used for being capable of language. Since books are normally not beings (I won't bring up the question of whether they are capable of language) they don't qualify as ghaH. You would have to use 'oH.
2) You can't use the pronoun in this sentence. In English we have adjectives and they sort of act like nouns. To show that a noun is described as another noun or by an adjective, we use the connecting verb "to be"/"is" in English. In Klingon pronouns can act as that connecting word to say that a noun is described as another noun, but Klingon doesn't have adjectives, so when you want to use an English adjective, the Klingon pattern doesn't work the same.
Words that have the same meanings as adjectives in English are usually verbs in Klingon. And because they are already verbs in Klingon they don't need something to act like a verb to connect them to the noun - they connect directly to the noun themselves! So Daj is defined as "be interesting" and doesn't take an additional "be"/"is".
You could say Dajqu' 'oH, but in that sentence the 'oH is just representing "it" and Daj represents the "is interesting".