Have Spacesuit -- Will Travel (1958) by Robert A. Heinlein

Wikipedia wrote:Have Space Suit—Will Travel is a science fiction novel for young readers by Robert A. Heinlein, originally serialised in The Magazine of Fantasy & Science Fiction (August, September, October 1958) and published by Scribner's in hardcover in 1958. It is the last of the Heinlein juveniles.

Heinlein made use of his engineering expertise to add realistic details to the story; for a time during World War II, he was a civilian aeronautics engineer working at a laboratory where pressure suits were being developed for use at high altitudes.

The title refers to both the expression "Have tux, will travel" and to the television show "Have Gun—Will Travel".

Have Space Suit—Will Travel was nominated for a Hugo Award in 1959. It also won the Sequoyah Children's Book award for 1961

Wikipedia wrote:Clifford "Kip" Russell, a bright high school senior with an eccentric father, enters an advertising jingle writing contest, hoping to win an all-expenses-paid trip to the Moon. He instead gets an obsolete, but genuine, used space suit. Though a few make fun of him, with the help of sympathetic townspeople, and using his own ingenuity and determination, Kip puts the suit (which he dubs "Oscar") back into working condition.

Kip reluctantly decides to return his space suit for a cash prize to help pay for college, but puts it on for one last walk. As he idly broadcasts on his shortwave radio, someone identifying herself as "Peewee" answers and requests a homing signal. He is shocked when a flying saucer, closely pursued by a second, lands practically on top of him. A young girl (Peewee) and an alien being (the "Mother Thing") flee from the first, but all three are quickly captured and taken to the Moon.

Their alien kidnapper ("Wormface") is a horrible-looking, vaguely anthropomorphic creature who contemptuously refers to all others as "animals". Wormface has two human flunkies who assisted him in initially capturing the Mother Thing and Peewee, a preteen genius and the daughter of an eminent scientist. The Mother Thing speaks in what sounds to Kip like birdsong, with a few musical notations in the text giving the flavor of her language. Kip and Peewee have no trouble understanding her.

Kip, Peewee, and the Mother Thing try to escape to the nearest human base by hiking across the lunar surface, but they are recaptured and taken to a base on Pluto. Kip is thrown into a cell, later to be joined by the two human traitors, who have apparently outlived their usefulness. Before they later disappear, one mentions to Kip that his former employers eat humans.