First a note on the poetry: all the above five star reviews have said it far better than I can. It's enough to say that if you hear the words "20th century welsh poetry" and immediately think Dylan Thomas, then you need to read this man, who is absolutely no relation. RS Thomas is one of the greats from the 20th century, and his reputation will only grow further in time.
It's said that around the time of his death that RS Thomas was being considered for the Nobel Prize. Take a look at this edition and it's easy to see why: he had an unflinching poetic vision, time and again returning to the biggest of themes, such as politics, our destiny as individuals, our destiny as a species, God, religion and so on. Make no mistake here: Thomas is a very, very weighty poet indeed. In fact I can't really read too many of these in a sitting without my head spinning. It's not so much that his poetry is a series of bitter yet necessary pills to be swallowed, but rather (and like hearing someone like Noam Chomsky speak) you know that you're in the presence of someone who is feeding you vital, perspective-altering information. What's more, Thomas had the ability to say so much in his own hard, terse way. Sometimes I wonder whether he was two parts fire and brimstone preacher and one part zen master.
My only gripe with this collection is that- as the title implies- it's now 21 years old and counting (published 1993; accounting for verse up to 90). The great man has been dead some years now, so I'm sure whoever holds the copyrights and administers the estate can get it together and re-release this book in updated form. In the meantime, don't stint on buying this: rather than going for a watered-down "Selected" edition, go the whole hog and get this plus any post-1990 works you can get second hand. Like Yeats, Thomas produced sterling work right until the very end and his later work repays the effort put into reading it.