One problem with the book is the use of dates. The dates I use above are the ones that stick in my mind, but they may in fact be wrong. A second complete reading of the book would no doubt serve to cement firmly in the mind such (to me) significant details, but I do not want to do that right now; there are other things to read. Dates are very important for people who have an interest in literature, just as they are for the student of history, because these markers serve to help form a personal geography of a writer, in the reader's own mind. They also help to fix the individual writer within a broader landscape of concepts, stylistic traits, and events. This broader intellectual and conceptual landscape is called 'understanding', and is the reason why we read books like this. The main failing of the book, then, from my point of view, is that this aspect of the poet's development has not been firmly enough nailed down within the book's confines.
But Westover has done many other important things. Thomas, his subject, is known for certain characteristics beyond the poetry itself. Welsh nationalism, for example, is one thing he is known for, but for most readers this part of his psychology will not be very interesting. Westover does make it clear that the times when Thomas is most nationalistic in his poetry is when his poetry is weakest. Which is revealing and of greater interest to the general reader than the fact that he thought about Wales, where he was born, in such a way. Jingoism is not very pretty, you might conclude.
Westover's point of departure seems to have been the combination of a love of Thomas' poetry with a dissatisfaction with the way that other people had mainly written about it, so the scholar goes back to the poetry and leaves aside for a change the biographical elements that had preoccupied others. Westover's deep knowledge of poetry in a technical sense makes him an ideal vehicle for such an undertaking; you feel as though you are in competent hands as you read about just how Thomas' poetry works at different times in his (remarkably long) writing life.
Because I write poetry, for me the most interesting thing to consider is how Thomas abandoned metre and rhyme for other techniques. While following closely on Thomas' heels, Westover does not interrogate this switch specifically. But that's no surprise because this is my personal area of interest, and not Westover's. Nevertheless, there are things I might say about how Thomas dealt with the problem of creating meaning, and about the tools he deployed in that pursuit at different times of his life. But to say these things would not only require a different vehicle than a book review. It would also require a more intensive reading of Westover's very valuable book.