A story about a superintelligent talking dog? It sounds terrible, like something out of a twee Disney film, but in actual fact Stapledon manages to avoid anything like that, and has written an incredible, touching story. It reminds me of "Call of the Wild" and "White Fang", and doesn't avoid the dark side of Sirius' nature... there are a couple of particularly savage passages where Sirius kills a sadistic farmer, and also "murders" a horse just to indulge his canine instincts.
Sirius ends up seeing the full range of human life, from bad to good, and more. He is also not a true dog, and finds himself not only alienated from human beings who cannot accept him fully (with a handful of exceptions), but other dogs who are like cretins to him especially his "lovers" (as the book puts it). Despite having difficulty speaking and writing (he devises ways to get around that), Sirius has an advantage over other dogs through his intelligence, and over humans in his hearing, sense of smell etc. What we get is not only a satire on English life during WWII, but an almost autistic view of the world, seeing everything but not able to integrate oneself into it.
Of course some of the writing is dated, and Stapledon at times takes a very colonial view of the Welsh and their language (Sirius is originally brought up on a Welsh farm by English academics). Some of the style is very dry and typical of the period (for example when Sirius spots a holy roller farmboy pleasuring himself, Stapledon calls it "something unspeakable". Fortunately Victorian hangovers like these are not common).