I’m not sure about textbooks specifically on infinity. As another comment mentions, the study of infinity generally falls under the math subject of set theory s

I’m not sure about textbooks specifically on infinity. As another comment mentions, the study of infinity generally falls under the math subject of set theory so a set theory textbook would be a mathematician’s definitive resource on infinity. I learned set theory from the book Elements of Set Theory by Herbert Enderton. I’d say that books alright, but would not give it my highest recommendation. It’s very dry and technical, and most of it’s not about infinity.