I'll be filling this one out over time. For now I will just start with two works:
Gods of the Ancient Norsemen by George Dumezil. Dumizil is best known for his theory that both Indo-European society and it's mythology were fundamentally Tri-partite in a way that aligned class and function and that this could be seen in various forms in all derivative traditions including as here the Scandinavians. In Dumezil's original formulation this was also hierarchical with the Priest/Magician/Judge at the top, the Warrior a step below, and the Farmer below that. My view is that this gives too much weight to the Indian Caste system and that at least in the European context the functions were organized on the same level but drawing their practitioners from different classes with the King representing all three functions: i.e. Judge-Warrior-Fertility God somewhat equally. My reading on this topic is out of date, suggestions for more current works welcome.
On the Celtic side I am going to put up T.F. O'Rahilly's Early Irish History and Mythology which is a series of essays on the history, linguistics and myth of Early Ireland, which here should be understood as Ireland before the Vikings (itself the title of a book I will link to soon) or basically Ireland prior to 800 AD.
I am going to set out and re-read both of these over the next few days but would be interested in hearing from anyone who had read either or indeed any book covering the same materials.