We have inscriptions from "Romanized" Belgae and we do have coins from them, but there are no literary sources written by the Belgae (let's not get into the discussion of what the term covers) that survived.

Non Roman sources on the Belgae that I can think of at the top of my head are limited to: Ptolemy (mostly simple geographic notes) and Strabo (geography and entography; which you can find in translation here: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0239%3Abook%3D4%3Achapter%3D4)

So when it comes to literary sources we are highly depended on Roman sources. To better make sense of these writings it is obviously necessary to understand the writers and their motivations (both using historical critique yourself and picking up secondary literature) and it is necessary to use critical editions of the texts.

A good book to help you understand Roman etnography, and thus helps you to understand the source material better could be Greg Woolf's "Tales of the Barbarians: Ethnography and Empire in the Roman West". Though any book from the author is worth a read.

Hope this helps.