In real mythologies the tales change and develop over time: names, times and places change with the constant retelling and embellishing by the story-tellers. In real mythologies, accounts often become fragmented and there may be several differing accounts of a single event. It's the same with the mythology of Middle Earth and Valinor. The way Tolkien develops the stories over the course of his life-time is analogous to the way real heroic events become legend and the legends become myths and all sorts of changes reshape them. Real life examples would be 1) the story of Beowulf who may have been a real but mortal hero of his people, but in the telling and retelling of the tale over the centuries, he was able to swim for 2 weeks in the freezing seas of Northern Europe wearing mail and he was able to do battle under water; 2) the Arthurian legends where the hero has been claimed to have lived and had his court at a dozen or more locations in Britain; 3) the Merlin legends where Merlin may or may not have been associated with Arthur 4) The Robin Hood legends where again, Robin has been claimed to have been based at various locations. And so on.
Lost Tales Part 1 is a collection of accounts of the origins of the world of Middle Earth and Valinor, concentrating mainly on the gods and the elves. The stories overlap those in Silmarillion but differ in many instances. Names, characters and places had been changed by the time Tolkien finalised the material that went into The Silmarillion. It's fascinating to see the character and story development with notes and commentary from Christopher Tolkien drawing attention to the changes and stating (where the reasons were known to him) and providing convincing speculations (where specific information was not known) why these changes were made.
This book will be appreciated by the person who enjoyed reading Lord of the Rings but was left wanting to know more, and partially satisfied their curiosity by reading Silmarillion, but was still left wanting to know more ....