Old lore is primary texts of note from ancient days, associated with a British-and-Northern (or, "plausibly of some kind of relevance") tradition. These are all fairly recommended if you want a strong mythological grounding or a sense of having Studied The Old Ways and Graduated from Pagan uni. It's good to have a familiarity with them, as a citizen of the world if nothing else.
Book | Mindblowing stuff. If you're used to the fragmentary and odd recordings of the Mabinogion or Eddas, you have no idea what's waiting for you. The Kalevala is a poem of the ancient myths of Finland, compiled from the folklore still present in the early 1800s. The clarity of the poetry perhaps this indicates it's been more generously edited, but it's still a work of wonder: creation of the world and ancient mighty ones wandering the land. A big influence on Tolkien, and just a delight to read. I have a new Penguin Classics edition (trans: Eino Friberg; ed: Jukka Korpela). I love it so much. Also, the metre used in my translation means you can sing it using Mike Oldfield's tune of the 'Song of Hiawatha' from Incantations
✪ Story Archaeology (2015-)
Podcast | Can't recommend this highly enough. Irish mythology is messy and contradictory; these two experts talk you through it, one story at a time - telling new versions of the tales, then exploring the language and history of each. Fantastic. Essential listening for your basic mythic education. Link.
Beowulf (975 - 1025)
Book | The most prominent surviving British text of the Anglo-Saxon period. I have J.R.R. Tolkien's translation, which is functional and direct - aimed at learners to be an accurate translation. There's also the recent Seamus O'Heaney poetic adaptation, which attempts to put the reader in the "mood" of the original text.
Like most of our lore, the poem is Christianised, with a sense that both audience and poet would have had a strong grounding in Biblical knowledge.
Things of note: Hereot, the Solar village under attack by strange things from the (Stellar) fen; Hrothgar is a Winter King, with a sense of strength and fading and loss rather than a hero and protector; the association with the wetness of the Sea and the Fen with creatures from outer darkness.
Book | A collection of ancient prose texts from Wales - stories of heroes and (presumably) gods. The Mabinogion is very odd, and might be at its most accessible in an adapted version (either for children or adults), where someone has pulled the threads of the tales out.
The Eddas (13th c)
Book | The mythological texts of the Vikings: in Poetic and Prose form - "The Poetic Edda is the modern name for an untitled collection of Old Norse anonymous poems, which is distinct from the Prose Edda written by Snorri Sturluson". England has forgotten its gods, but as far as we can make out the Anglo Saxon deities such as Woden had something in common with the Viking deities as set down in the Eddas (even if many myths and specific local variants have also been lost). You had best look out some advice from the Heathen or Asratu communities for how to approach reading these. I have a nice modern penguin edition of the Poetic Edda, but I found it basically unreadable - it's allusive and vague. I did a lot better reading a nice adapted version, summarising the stories and characters; and then going back to the Poetic Edda with a sense of what was what.
The Seafarer and the Wanderer (10th c)
Book | Melancholic Anglo Saxon poetry; influenced my vision of the Winter King, and his dour memories of fading glory