Obv doesn’t have to be same book. Just looking for an up to date book on Boudicca, not a modern translation of Tacitus. Also, a book on the druids. Source material would be fine, but Romans being Romans, probably not accurate. Druidic culture, practices…though again, non written material culture, I’m sure it’s hard to find accurate reporting.
Before going into book recommendations, you might be interested on these previous answers on the topic. (And as there still more to be said on these, there's still several answers and recommendation to be proposed)
How much do we know about the historical Boudica, and how much is hagiography/myth-making in later days? by u/iguana_on_a-stick and u/Tiako
Sources on female Druids by u/Kelpie-Cat
Sources on female Druids by u/Libertat
Primary sources
Now, primary sources are indeed scarce and indeed, giving the lack of written formal literature in Iron Age Britain (and generally, non-classical Iron Age Europe) we have to rely on Greek and Roman sources. Rather than outright making stuff up, they most of the time tend to be merely tendentious or incomplete as focusing on exoticism or their own cultural considerations : keeping that in mind, they're still usable and unavoidable as main sources.
On Boudica
On Druids you'd have, among many scattered mentions (both chronologically and literary)
Secondary Sources
Ronald Hutton; Blood and mistletoe - An history of druids in Britain; Yale University Press; 2009 Not so much a book on ancient Druids in Britain (or rather, not just that) as they are essentially, and conspicuously, absent from ancient sources, than a book on how they became a staple of ancient Britain depictions trough the ages and fitted into what we know of it.
Richard Hingley, Christine Unwind; Boudica, Iron Age warrior queen; Bloomsbury Publishing; London; 2006 It is a particularly interesting read, not only in depicting the queen as she was in ancient sources, but also confronting this this figure to archaeological elements and posterior illustrations during Middle-Ages, Renaissance as well as Victorian or Contemporary periods.
Caitlin C. Gillespie; Boudica: Warrior Woman of Roman Britain. New York: Oxford, 2018. Probably the best recent publication you'd have on Boudica not just as an ancient figure of rebellion against Romans, but as far as we know in herself as a feminine power figure in British and Classical antiquity.
Barry Cunliffe; Druids: A Very Short Introduction; Oxford: Oxford University Press; 2010
Jean-Louis Brunaux; Les Druides, des philosophes chez les Barbares; Points : Éditions du Seuil; 2006 An important aggiornamento of what we know of Druidic presence in Iron Age Gaul trough ancient sources and archaeology. The author goes strongly (maybe a bit too strongly) against the traps of comparativism and 'celticization' of the Druids in favour of placing them into their known context on the ancient mainland (and their relative absence elsewhere).
John Davies, Tom Williamson (eds.); Land of the Iceni: The Iron Age in Northern East Anglia ;Centre of East Anglian Studies; Norwich; 1999