To van Vogt, the universe is a violent place and teaming with life. The Space Beagle is a scientific exploration ship sent out from Earth to go where no man has gone before and study all that it finds. The alien beings that are encountered are invariably hostile or if they aren't their efforts to communicate their friendship inadvertedly cause much harm.
The book has an episodic nature that arises from the fact that it was forged from four seperate short stories that he reworked into one story.
The narrative is mainly from the perspective of Elliot Grosvenor, who is is the lone representative on the ship of his particular scientific field; Nexialism. The science of whole-ism, an integrated approach to all the different scientific fields. Grosvenor, and his new science generally, is initially looked down upon by the other scientists and the meta story that unifies the book describes the politics on board and how he gradually overcomes prejudice to gain acceptance amoung his peers.
The book is heavy on science and ideas, pretty visionary for it's time altough some has innevitably dated. Once particular scientific notion that is often called upon to help them overcome an alien menace is the cyclic theory of civilization and one wonders if this is quite an old fashioned notion by today's standards?
The depiction of the alien creatures is deeply imaginative and the excitement and tension of the book never lets up. One might be inclined to criticise the book for it's fairly flat depiction of characters but to my mind this shouldn't detract from the fact that read as SF, it ticks every box. If you're not ordinarilly a fan of SF, this is probably not going to convert you. But if you are, you could do far worse than reading this outstanding work of SF.