I generally liked this book and I definitely recommend it for its view of humans as just another species. However, in my view it has two weaknesses.
Firstly, the author has a tendency to descend to political and social commentary, which tends to come across as a bit dated at times. In some cases it feels more like a view into the culture of 1967 than any profound insight into human nature. For example, he asserts that the "psychological damage" done to our territorial nature by "rows of uniformly repeated, identical houses" is "incalculable" - surely a social comment, not a scientific view. Nuclear war and population growth are mentioned more than once as real concerns - again, his analysis is a reflection of the time and place that the book was written. (He may well be right about his theories of course, and probably is in many places - it's just quite subjective).
Secondly, quite a lot of the book reads like a "just so" story. <Just So Stories (Wordsworth's Children's Classics)
>. E.g. when discussing how humans feed, he describes how we like a few well-spaced meals rather than continual grazing. This is held as an example of our carnivorous ancestry. On the next page however, he describes how we do sometimes eat (sweet) inter-meal snacks. This doesn't present any problems though: hey presto! in this case we do it because of our primate ancestry. If you follow the same line of reasoning, you could conclude that we like sunbathing because of our reptile ancestry.
The chapter on animals seems the worst in this respect, sometimes almost laughably so. For example, he analyses, in detail, the results of a survey of eighty thousand young children who voted for their most and least favourite animals. From the list of the top ten favourites (with chimps and monkeys in the top two slots), he concludes that there's a strong bias toward preferring anthropomorphic features. Maybe. However, the list also includes horses, elephants, lions and giraffes; and I find it hard to see how these particular species are particularly more anthropomorphic than any others. (He decides elephants are popular because they have trunks - just so).
However, regardless of the criticisms above, I would still recommend the book as genuinely interesting and full of thought-provoking ideas - but I suggest you read it with a skeptical eye open.