Reddit mentions of Decline & Fall of the Roman Empire (Wordsworth Classics of World Literature)

Hey :-)
It depends on what you mean by book (as in to purchase or if you have a University Library that you could grab from). I'll assume you want to purchase one (if not, let me know).

Here is a short list of works that contain some great information about Diocletian (but are not solely about him):

Gibbon's Decline and Fall - it has it's issues but they are easily moved to the side, this work has been probably the most influential with regards to this era of Roman History (This is not exactly the most fun read, being over 1000 pages, but it is too important for me not to mention)

http://www.amazon.co.uk/Decline-Empire-Wordsworth-Classics-Literature/dp/1853264997/ref=sr_1_1?s=books&ie=UTF8&qid=1409976827&sr=1-1&keywords=edward+gibbon

For a shorter read Averil Cameron's The Later Roman Empire is a great introductory work and has a good chapter on Diocletian (as well as the Tetrarchs in general), an advantage of this book is that is costs like £0.40 used so even if you hate it, you've not invested in it much!

http://www.amazon.co.uk/Later-Empire-Fontana-History-Ancient/dp/0006861725/ref=sr_1_1?s=books&ie=UTF8&qid=1409976976&sr=1-1&keywords=later+roman+empire

Otherwise any really good scholarly books I can think of are more of a University book and cost around £25+ each. I have a good list of books like this if you have a want for them :-)