When I was little, I had a couple of big compendium books I really loved. One was A Treasury of Literature for Children, which was a collection of excerpts from longer novels and poems (the story of Rikki-Tikki-Tavi from The Jungle Book, an extract from What Katy Did, 'A Smuggler's Song' and so on). King Arthur: Stories of the Knights of the Round Table, a collection of various stories relating to Arthur and his knights, was the other.
When I read it, though, I always skipped the first chapter. The second chapter told the story of Merlin's childhood and the third finally got to the birth of Arthur (and therefore, to me, the interesting bit) but the first explained how the kingdom of Britain was, in fact, founded by Trojans. This chapter, I'm afraid to say, did not interest me at all!
The story given in the book (which I presume became attached to Arthurian legend somewhere in the medieval or early modern period, though I don't know when) is that Ascanius' grandson Brutus was exiled after accidentally killing his father and was led to Britain by Artemis through a dream. However it got attached to Arthurian legend, the intention is clear - to make Arthur a great Classical hero as well as a British one and to tie the Britons, like the Romans before them, to Homeric legend. Julius Caesar's invasion of Britain is reimagined as a meeting of ancient kinfolk and Arthurian legend is made part of a larger Classical story.
Going back to it as an adult, the chapter still feels a bit rushed, but works much better. Artemis, orders given in dreams, people accidentally shooting relatives and going off to found new kingdoms all seems much more familiar to me now and the tying together of the two mythologies is quite fun. I've no idea if this book is still in print, but I'm glad I still have it - it's beautifully illustrated by Jan Cerny and I always enjoyed its retellings of the Arthurian legends as a child. I'm glad it includes the Classical sections as well, as I can really appreciate them now - no more skipping Chapter One!