There are two stories at work here. Both can be comfortably read as separate entities.
The middle sections, known as Book 1 & Book 2, deal with the tale of Thaw in a truly fantastic piece of semi-autobiographical work. The many moments of raw, wide-eyed genius make it a joy to be involved with. 9/10
The first and last sections deal with the fantasy of Lanark. It has its interesting moments but proves frustrating in the extreme as characters and landscapes change at Gray's, headache inducing, whim. 3/10