Alan Garner is a marvellous wordsmith, who's turn of phrase is a blend of the modern readable and the ancient - just one line finishing a tale here for example
"and of her he was as glad as grass would be of rain" is worthy of ee cummings.
This is not all his own work - there are re-tellings here - and what could be wrong with re-telling a folk tale in your own words? that is the very nature of the art.
The tales (as pointed out correctly by another Amazon reviewer) are not all British at all - there are India, Japanese and North American amongst the stories, and this lends a depth to common elements (magic, devils, faeries) as well as a richness to the voices that the writer must "speak" with. For me, I liked all the voices, but some might find the refrain (or repetititous phrases) of old norse rendered through 21st century English a little annoying. This is not the Owl Service or the Weirdstone of Brisingamen...it is more like Garner's more recent work, but of course, he is, as he says in the brief introduction, trying a double trick - re-telling oral tradition in writing, and re-imagining it as if his grandfather (a smith) was reciting it to the current author as a child. This works well in some stories, and is less visible in others.
I would suggest reading this aloud to your children to see how well he has succeeded. My kids are to old so I had to pretend, and it worked very nicely for my inner child.
Oh, and there is a smattering of poems here - I thought they were superb, and alone worth getting the book for.