"The Beast" by A.E. van Vogt

Offers the story of a flawed hero possessing almost superhuman strength. When his wife is kidnapped, war veteran Jim Pendrake embarks upon a search that takes him to a lost colony on the moon--and a secret, sinister society. Originally published in Astounding Science Fiction.

If I say too much about the plot of The Beast it will sound ridiculous. Maybe it is. Suffice it to say there are underground Nazis involved in a nefarious conspiracy, immortal polygamous cowboys, an interplanetary matter transporter, a million year old genius Neanderthal named "Big Oaf", and that most of it takes place within millennia old caverns created by an extinct alien civilization on the moon. With all of that and more van Vogt manages to harness his extravagant imagination and inimitable style to tell an amazing adventure story full of action and intrigue, and at breakneck speed. Oh, and there's a saber tooth tiger.

Seriously, what other author, then or since has so consistently and unabashedly unleashed his/her imagination to such an extent? van Vogt straps a rocket to his imagination, which either explodes after launch or takes you all the way to the moon and beyond. (GoodReads)