The Eidolon Chronicles-Book 1-The Secret Country by Jane Johnson
All Ben wanted was two Mongolian Fighting Fish. What he got instead was a small cat who talked to Ben in a gravelly voice. What was even more amazing to Ben was the fact that now he could also understand the neighborhood cats as well.
According to the small cat, whose name was Ignatius Sorvo Coromandel but went by Iggy, Ben isn’t entirely human, just mostly. When Ben finds a sprite called Twig, Iggy tells him they must take the sprite to Eidolon, the Secret Country and they have to go by way of the Wild roads (magic highways) or the sprite would die. Ben won’t leave his mother, who is sick but helps Iggy and the sprite get to the door to Eidolon.
Ben finds out that someone is stealing creatures from Eidolon and selling them to humans as pets. The human world is bad for them and they need to get home. Ben also finds out that his mother is the elfin queen of Eidolon and he and his sister are half elf and a prince and princess.
With the help of his sister and a host of magical creatures, Ben is able to stop the selling of magical creatures, at least for a while and brings his mother back to good health.