This question is not very specific. But whenever I read about (later) Rome and the Early Medieval period, I'm fascinated by the huge number of Germanic tribes, which are often mentioned without much detail. The Cimbri; the Teutons; the Suebi (or Suevi); the Chatti and Tencteri (mentioned by Tacitus); the Alamanni; the Goths (Visi- and Ostro-); the Tungri; Pliny's Vandili, Burgundiones, Varini, Carini, Gutones, Ingævones, Chauci, Istævones, Hermiones, Hermunduri, Chatti, Cherusci, Peucini, Basternæ, and Daci...
It goes on and on. But I almost never see much detail about what distinguished these peoples from one another. Do we know anything? What do we know? What are our sources?
Any type of reading would be appreciated but some books for a layman would probably be most helpful.
Tacitus is a good start point. Many of these tribes didn't leave written records of themselves, so we're left with things like Tacitus' Germania, which is a catalog of peoples encountered by Romans, and some would argue that the Germania was actually more of a satire of Roman society than an honest ethnography (i.e. something closer to Thomas Moore's Utopia or Jonathan Swift's Gulliver's Travels than to a BBC or PBS documentary).
For historians who want to talk about Germanic tribes, this is a bit of a problem. We know how outsiders described them, but we have few clues as to how they described themselves. Basically, what distinguished one Germanic tribe from another was the ability of Roman elites—like Tacitus—to treat them as different groups. There's some important clues that the Romans really didn't have it all figured out. While a single source like Tacitus might make everything look neat and clean, when you read sources from different periods, you see that hierarchies and relationships changed.
It gets even muddier when you try to read 19th or even 20th century histories about the Germanic tribes. During the heyday of nationalism (which probably isn't actually past), people used to claim that they shared geographic and biological ancestry with some of these tribes: Germans, Franks, Saxons, Goths, Langobards, etc. Historians influenced by this line of thinking were often quick to accept that the German tribes were as distinct and timeless as authors like Tacitus tried to portray them.
As the 20th century wore on and ethnic conflicts tore old empires apart and led to atrocities of holocausts and genocides, scholars became more cynical about ethnicity, questioning whether things like dress, custom, and lifestyle are really something that we're born with, or whether it's something that's learned, adopted, and adapted over time. Now many historians are skeptical that the Germanic tribes were really as distinct as Roman authors wanted them to be. After all, their thinking goes, the Romans were colonizers, and organizing indigenous peoples into simple societies makes them easier to understand and presumably to manage.
In terms of reading recommendations, the first three are top notch. One discusses Tacitus, another the early medieval texts describing Germanic tribes, and another the more general issue of Germanic ethnicity as described by late Roman authors. If you get interested in how scholars have debated ethnicity, the fourth book would be a natural next step. Cheers!
Christopher B. Krebs, A Most Dangerous Book: Tacitus's Germania from the Roman Empire to the Third Reich (2011).
Walter Goffart, The Narrators of Barbarian History (A.D. 550-800): Jordanes, Gregory of Tours, Bede, and Paul the Deacon (2nd edn., 2005).
Walter Pohl, "Telling the Difference: Signs of Ethnic Identity," in Walter Pohl and Helmut Reimitz, eds., Strategies of Distinction: The Construction of Ethnic Communities, 300–800 (1998), pp. 17-69.
Andrew Gillett, ed., On Barbarian Identity: Critical Approaches to Ethnicity in the Early Middle Ages (2002).