Suggest Pagan Celtic Britain by Anne Ross.

Keep in mind that we base a lot of our "knowledge of history" on very little information, quite often a few lines on scraps of paper. Druid history, I believe, is based on three short lines of contemporary notes and the imagination of a later period. Picts are almost in the same boat.

Ross's book details the time around them and the tribes that may have been them. It's a thick tome but honest. My favourite is when, after describing what some see as a pagan fertility Goddess scratched in a chalk cliff face, she acknowledges it may have been a pornographic drawing scratched out by a bored guard.