Reddit reviews The Decline and Fall of the Roman Empire (Volumes 4-6)

If you are really serious:

http://www.amazon.com/Decline-Fall-Roman-Empire-Everymans/dp/0679423087

and

http://www.amazon.com/Decline-Fall-Roman-Empire-Volumes/dp/067943593X/ref=pd_sim_b_1

https://www.amazon.com/History-Decline-Fall-Roman-Empire/dp/1470067099/ref=tmm_pap_swatch_0?_encoding=UTF8&qid=&sr=

https://www.amazon.com/gp/offer-listing/067943593X/ref=tmm_hrd_used_olp_sr?ie=UTF8&condition=used&qid=&sr=

I did not see books 4-6 in paperback

The author is Edward Gibbon
This set is no longer accepted as reliable source materials. (apparently some "facts" are just made up)

I recommend books from Peter Connolly they are well done and approachable with a lot of great illustrations.

https://www.amazon.com/Peter-Connolly/e/B001IQZOLA/ref=sr_tc_2_0?qid=1486396652&sr=1-2-ent