I'm sure this has been asked before, but it's strangely difficult to find good work on the transition periods between historic eras - ie the experience of people during the decline of Rome, or the beginning of the end of feudalism. These are the messy periods of history where little synthesis exists, but surely someone must have at least collected primary sources from individuals who realized that something big was happening, and in hindsight they were witnessing the birth of the Arabic Golden Age or the Industrial Revolution, etc.
Any leads greatly appreciated!
I'm not really sure I could point you to any good books that discuss eras of transition on a theoretical basis. I do know that the historical journal Continuity and Change is basically devoted to interdisciplinary historical sociological essays on these topics and might be of interest.
To specifically discuss the study of the era of the "decline of Rome" this is part of what's called "Late Antique" studies which was basically invented to cover the rather messy period of transition between the classical world and the medieval world. It's an oldy but a goody but I would recommend Peter Brown's "The World of Late Antoquity" as a good place to start. Peter Brown and Glen Bowersock are two of the big names in late antique studies, so I would look at their work generally.