There are a number of interesting books and chapters on the Picts in various books on early Scottish history.

One good one is in the Historic Scotland series, which I found at one of the many historically significant sight I visited in Scotland, back in 2000.

Another good one, which is also EASILY READABLE is the volume PICTS in the Osprey series. Many of these volumes are written by the very best historians on the subject, are concise, yet detailed, and readily accessible at MANY bookstores, which many of the best Scottish books are not (at least in the former Colonies).

By the way, be hesitant of many books concerning the original form of Druidism. I have sought many in an effort to understand it, especially to better understand the beliefs and culture that Colum Chille and the other Irish/Celtic Christians faced in evangelism in early Scotland/Pictland and how it then influenced the monastic based Celtic Church. In all I have read, the historical basis is VERY SHAKY and is often a "new age" implantation upon a single stone of reference in a Roman document. Then much weight is placed on this assumption.

An example, one book makes a great deal of effort to elaborate upon Pictish use of fishes in standing stones, even describing details of ceremonied with NO, NONE, NADA references to any known historical document of anything else. The only reference is the very PRESENCE of fish carvings on standing stones.
The problem is that a man named St. Samson was referenced ny the Celtic Church as going to Pictish sacred sights and "Christianizing" them through ceremonies and the CARVING OF FISHES upon them, as a symbol of Jesus the Christ (this the Christian Fish seen everywhere-at least in the "Bible Belt" South- today).
The truth may be here, or maybe the story was told to explain the presence of these fish on the stones.
The point-> Such a basis for understanding Pictish beliefs can be VERY SHAKY. All that survives is written by thise who sought to KILL them (Romans) or to CHISTIANIZE them (the Scotti of Dal Raidia).

That said, the above two books are a decent basis for general understanding, as much as such can be, and the basis for further research.