The Picts, a history, by Tim Clarkson.
This is supposed to be a narrative history overview of 600 years of the Picts. The author never really commits to the narrative form though, so it ends up as more of a chronological overview of the sources with some evaluation of their reliability. I was quite glad he did that- it wasn't a hugely gripping story but i feel like i got a lot of information. It's not an exhaustive and detailed survey though, still readable if a bit on the dry side.
Because of the sources, it's very focused on royalty and battles. There's little about who the Picts were and how they lived. Maybe we just don't know and need to do more digging.
There's also a good bit covered about other early mediaeval people in what's now Scotland and nearby, but only to the extent they interacted with the Picts.
I would recommend this if you want to know stuff but not if you want entertainment.