The ontological argument for the existence of God is the subject of the aptly title book by Jonathan Barnes The Ontological Argument.
This book is not for beginners in this topic. Barnes investigates the ontological argument from a professional standpoint, and makes extensive use of symbolic logic in his investigations.
For those of you not of a philosophical mindset, this book will be excruciating. Barnes spends much time investigating the nature or the word, or the state, or the word “is.”
This sounds impossible remote, but an investigation into the nature of existence, being, is-ness, is essential for an understanding of God in the context of the ontological argument. For the argument is trying to prove that God is, the God exists. Therefore, a great deal of semantic and logical hairsplitting is essential.