Just to be sure that you are looking for the book you remember (the hardest part of finding a book is knowing it's title).

Day of the Ness is about a boy and his cat. They live in a fairly remote area near a UFO landing site. Well maybe not so Unkown. The area has been marked by the boys father as a know alien settlement, but no one has been able to make direct contact, short of some trade goods being exchanged. The boy and cat go adventuring, and meet up with the "gotchas" who look like small water rounded cobble rocks with long spidery arms (good drawings). The two adventurers are invited into the underground settlement to rescue the "big think" (looks like a TV set with legs), from the evil aliens. After a long day's adventure, the boy & cat come down off the mountain.

My favorite part of this book is the cat. Who displays un-cat like behavior in that he stays with the boy, rather than going off on "cat business".