The Voyage of the Space Beagle

I read this as a teenager - back in the early 70s - and was immediately and deeply impressed. In this linked series of stories, van Vogt managed to create one of the best-realised aliens I had ever met until then ("Coeurl, a starving, intelligent and vicious cat-like carnivore with tentacles on its shoulders..."), and also inculcated in me a desire to do as the hero had: that is, think outside the box, and come up with unorthodox solutions to immediate and practical problems.

Still a book I would recommend to anyone.