The book that got me interested in sci fi as a youth was A.M. Lightner's Star Dog, first published in 1973. The book was probably about a year or two old when I read it. I picked it out of the science fiction section of the lovely wood and brick library at Lewis & Clark Elementary School in Great Falls, Montana.
Teenage boy Holton has a collie, and she gets loose the same night that strange lights are seen in the sky. Holton finds a dead dog in the road that night -- one with odd eyes, and luxurious fur, and... are those extra limbs? His friend Willy claims its an alien, buut the government takes the body away, claiming it is diseased. Eight weeks later, Holton's collie gives birth to a single pup, whom he names Rover (because his collie roved about conceiving him). The mother dies soon after, but Rover seems special. He has a luxurious coat, and seems to stare at Holton -- as if he were listening, understanding what he said. Rover seems capable of performing any trick, so long as Holton thinks about it clearly (and Rover isn't hungry).
And then one day, Rover speaks...
In some ways, Star Dog is a typical boy-and-his-dog story. It also has a very strong resemblance to E.T., in that the government is hot to get the dog and dissect it for its secrets. There's also a bit of Pinocchio, in that some unscrupulous carnival owners want to kidnap the dog so it can perform for them.
Lightner is forgotten today, but she wrote some 20 sci fi books for young adults. She spent the last 30 years of her life (she died in 1988 at the age of 83) writing nature books, each with the title The Biography of A [insert animal name here], for which she won some acclaim. In the 1930s, she'd been a member of the American Communist Party, and many of her sci fi novels of the late 1950s and 1960s contained strong elements of feminism and racial integration. (In Day of the Drones, a black woman comes to a planet where white males are enslaved.)
I liked Star Dog immensely, because -- as a kid who was intensely aware of his homosexuality from a very early age -- I, too, longed for a secret friend with super powers who'd make me feel special, secure, and accepted the way Rover made Holton feel.