Changing language of Books in books app

The language settings for iOS don’t alter the language of the ebook files themselves. The language of the ebook is set in the download of the file you got when you purchased it. So if you purchased a Spanish language version of an ebook, then it will only be readable in Spanish. To read it in English, you’d need to have specifically purchased an English language version of that ebook.

Ebooks are static in terms of language so you cannot translate an ebook on the fly with a software setting. If you purchased a Spanish language ebook, it is only in Spanish. Many ebook stores do offer alternate language downloads when purchasing if that author/publisher offers alternate language versions of their ebook.

Ileanainmexico wrote:

I can’t understand why all titles in Books are displayed in Spanish

I think it's because the store you are using is for Mexico and the language of the books offered is tailored to that. If you want to change the store you are using, check

https://support.apple.com/en-lamr/HT201389

If you search by a title in English, doesn't the book appear?