Sci/fi trilogy: creatures killed by hitting their noses, boy riding a hot air balloon on the cover

Received via email; I'll be delighted to be able to pass on any suggestions. Thanks! - Jeremy

looking for a sci fi trilogy i read as an 4th grader in the late 1960's. about a boy trapped in another world trying to get home and fighting against creatures who can only be injured by hitting them under their noses. one book has a picture on the front cover of a boy riding a hot air balloon.

any help?

John Christopher's Tripods series? The third book The Pool of Fire involves a bombing mission by balloon. (Not on another world, but the characters are enslaved inside a xeno-formed dome)

Almost certainly the Tripods series. The aliens depicted in that can be killed by a blow to a sensitive spot on their heads.