Diamond is writing this book for a general, broad readership, not a narrow readership of academics. That would have been difficult in any case, because he touches on so many different disciplines in the work. The work targeted curious, educated (but again, not specialist) readers who had an interest in the subject matter, which spans history, anthropology, evolutionary biology, and many other fields. The book was published by a large, commercial publisher (W.W. Norton) and won the Pulitzer Prize, which is not normally awarded to narrow academic monographs. By writing the book, Diamond sought to advance people's understanding of science and history and to sell books (always a goal of a book written for a major publisher.) But at the same time, the book exposed many general readers to academic theories that they would not have encountered elsewhere. He also hoped (a goal he makes quite clear) to change people's understandings about such important matters as race, culture, and "civilization," as well as the often unfortunate effects of European contact with the rest of the world.

This book is written for an audience that is not really expert in any field, but which is at least interested in the questions that Diamond is posing.

In this book, Diamond touches on issues that come from academic fields ranging from microbiology (how and why germs cause the illnesses they do) to political science (how different types of political organizations emerge).  This means that the book cannot really be aimed at people who are expert in any given field the way a book could be if it really just touched on one subject.

The only real prerequisite for reading this book (other than being able to read at at least a high school level) is interest.  You have to be interested in why the world has turned out the way it has and, specifically, why Europeans and their descendants came to dominate the world.

So this is a book aimed at anyone with a decent level of education and an interest in the major issue that the book explores.