Ever since we played our annual Memorial Game in Honour of a great mate, Mick Evans, way back in January 2017 I have been wanting to learn more about both Offa and his dyke. The link has all of the puns so I will leave them well alone!
This one has just arrived:

The Pen-and-Sword series of books can be a bit hit and miss, but I found this one excellent.
The book covers Offa’s thirty year reign and the 8th century rise of Mercia’s power to be one of the dominant Anglo-Saxon Kingdom’s. The book also describes how Offa was able to build in the work of his predecessor, Penda, who was able to bring the tribes of Middle England together and develop a militaristic warrior culture.
I fully recommend this one if you are interested in the early Anglo Saxon period.