Tales From the Flat Earth was reminiscent of Tolkien's Silmarillion in that it was less of a consistent narrative and more of a cycle of tales, a bible if you will, telling a story that spans many mortal lifetimes. Night's Master is Azharn, the demon king, who loves humanity because he can torture it; Death's Master is Ulume, the personification of death, who is utterly dispassionate, until a proud queen and her demonic "son" (read the book and you'll understand the scare quotes) nearly put him out of a job; and Delusion's Master is Chuz, the cunning and fickle personification of insanity. The conflicts of these forces against humanity and each-other shape the destiny of humanity even has humanity struggles to find it's own fate.
In my mind, Tales From the Flat Earth has two main points to recommend it:
Firstly, Tanith Lee is a very good writer. As a wordsmith, she is superb. Every word, every phrase, serves to create a wide variety of moods, whatever mood she needs for the story she is writing. The stories in Tales From the Flat Earth are gritty, opulent, sexy, epic, and personal, one at a time and all at the same time. From having read some of her other work (the Claidi Journals, her Young Adult series - here's a link to Book One), I can tell you that creating strongly evocative moods through a highly flexible style seems to be Lee's strength in general.
Secondly, the very idea of a themed anthology of mythological tales tickles me. It's an approach to fantasy that I've never taken in earnest, though I've occasionally played with it - my hard drive is littered with unfinished cosmogonies and theogenises. Perhaps I'll have to try my hand at it some time soon.
If Tales From the Flat Earth has a weakness at all, it's this: the style might not appeal to everyone. Although the narrator does delve into the thoughts of her characters, she is more distant than one expects from a modern novel. As I wrote earlier, Tales From the Flat Earth more closely resembles a cycle of myths than a novel, and if that doesn't interest you, you'll probably find the book frustrating.
But if I were you, I'd give it a shot. All the cool kids are doing it.
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One other note: Tanith Lee has something against the color blue. Blue poison, evil blue dead chicks... it never ends. So I suppose if you are a hardcore, militant blue fan, that might be a downer.