As someone actually in academic philosophy rather than history, I'm excited to finally be able to answer a question.

If what you want is a reliable, thorough account of Diogenes's life, or a deep exploration of his psyche, then I'm afraid that no such text exists. Most of his life was not recorded and all of his writings have tragically been lost.

If what you want is a fairly short but robust account of Diogenes' life, your best option is probably Diogenes' single chapter in Lives and Opinions of Eminent Philosophers by Diogenes Laërtius (no relation to Diogenes the Cynic, ironically). Although this book is pretty controversial, particularly because Laërtius does not verify his claims, it's probably the best option you have for the sort of text you describe. It can be found for free on Project Gutenburg. I'd say it's pretty accessible, but I'm the sort of person that often enjoys dry theory stuff, so I'm probably not a good judge of that.

If what you want is an account of Diogenes' philosophy and cynic philosophy in general, then there's a lot more available to you. I would recommend

The Cynics: The Cynic Movement in Antiquity and Its Legacy by R. Bracht Branham and Marie-Odile Goulet-Cazé

A History of Cynicism - From Diogenes to the 6th Century A.D. by Donald R. Dudley

and Diogenes the Cynic: The War Against the World by Luis E. Navia

A lot of these are pretty unapologetically academic and can be pretty dense. I would recommend them to anyone interested in ancient Greek philosophy in general and cynic philosophy in specific. Definitely keep them in mind if you want to read more.

Sorry if I made any mistakes, English is not my first language.