Unfortunately, like a number of topics, the pop-culture and occult interest makes it difficult at times to discern what's a legitimate academic work and what's just fluff - and even in academia, references to the Druids themselves are often embedded in studies of Celtic cultures, religion, archaeology (monuments, megaliths, material remains, etc.), literature, language (Ogham being a major point of focus), etc., often divvied up by national or regional area of interest (Ireland, Scotland, Wales, Cornwall, Brittany, Gaul, pre-Roman and sub-Roman Britain, etc.)

Blood and Mistletoe: The History of the Druids in Britain (2011) by Ronald Hutton is probably the most comprehensive and accessible work for a lay audience, and if nothing else Hutton's bibliography is a good starting point. You might also look at Caesar's Druids: Story of an Ancient Priesthood (2010) by Miranda Jane Aldhouse-Green, which focuses more on historical druids, with close examination of Caesar's accounts.

Sorry if just asking for sources without a specific inquiry is against the rules.

Asking for sources is absolutely fine here.

This is certainly not my area of expertise, however, having written on the Celtic wars with Rome I recall one or two sources that may be of use. The first is a historiography work by AL Owen entitled The Famous Druids: A Survey of three centuries of English literature on the Druids. Another is The Philosopher and the Druids by Phillip Freeman. This book is more of a chronicle about Posidonius's work. On that same note, Posidonius's works, although in pieces, are some of the few primary sources available.

Though it's older than you requested, written in the mid seventies, The Druids by Stuart Piggott might be something that interests you.