Hamish Haswell-Smith's "Scottish Islands" comes pretty close to being the perfect book for the area. You should be able to get it from Nick's Amazon link.
Of historical interest there's Martin Martin's "A Description of the Western Isles of Scotland (1703)", if you can lay your hands on a copy. Our edition was published in the 70s or 80s by James Thin in Edinburgh. Unfortunately, his bookselling and and publishing business diappeared a few years ago after his death. There's an online version
here.