New English Library science fiction

Jesse and Daisy and their dragon friend, Emmy, try to recover a Thunder Egg from merpeople who stole it from Daisy near the Inn of the Barking Seal, where the cousins are visiting their grandmother Polly.

"Many worlds of the Humanx Commonwealth boasted of 'natural wonders' but Horseye was truly unique -- the planet had the most spectacular river valley anywhere in the known universe and was home to three alien cultures. The fascinating planet just cried out for proper study, and after months of impatient quarantine Etienne and Lyra Redowl had finally received permission to begin a voyage of exploration to the source of the River Skar, a mere 12,000 kilometers Upriver. Old hands at cracking new planets, the Redowls studied the aliens languages, took local guides, and provided for emergencies. But nothing could prepare them for the awesome treachery of the natives or the unbelievable natural obstacles. And not even the natives understood the planet's deepest secret..." From Goodreads

On the face of it Santaroga seemed like any other small California town in an agricultural valley. But this one had its own — unofficial — rules. Outsiders found no houses available to let or for sale. G.I.'s always returned there on discharge. No Santarogan ever left the valley. No chain Supermarket could open there. Were they simply "last-ditchers"? Or was there some other explanation, physical, psychological or even extraterrestrial? Gilbert Dasein, pshychologist, with an old college girl-friend living there, went to try to find out. It wasn't any of the things he thought it might be and some of his investigations led him into strange paths before he was able to pass through The Santaroga Barrier.