Will I Be Sinful If I Don't Tell Others About Books Containing Allah's Name?

Walekumussalam,

(1) No. If you have already given the book and you did not intend any disrespect, then you are not sinful. However, as a matter of caution and proper etiquette, it is better—when possible—to briefly mention that the book contains religious text and should be treated with respect.

(2)  If you get the opportunity, it is good to mention it as a piece of advice. However, it is not a religious obligation upon you. Your main responsibility is simply that you yourself do not become a cause of disrespect.

 (3) No. It is not obligatory for you to search through all the books or inform everyone. Islamic law does not burden a person with such unnecessary difficulty.

Advice :

In such matters, falling into excessive doubts and worries is not correct. Islam is a religion of ease. It is not appropriate to burden yourself with responsibilities that are not actually obligatory upon you and keep your heart troubled. It is better that whenever you get the chance, you may advise about maintaining respect, and then keep your mind at ease.