There are 2 books in the The Fourteenth Goldfish Series series. The newest book is The Third Mushroom which came out in 2018.

1
The Fourteenth Goldfish

Book cover of The Fourteenth Goldfish
Believe in the possible . . . with this "warm, witty, and wise" New York Times bestselling novel from three-time Newbery Honor winner Jennifer L. Holm. A perfect read about a child's relationship with her grandfather!

Galileo. Newton. Salk. Oppenheimer.
Science can change the world . . . but can…
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2
The Third Mushroom

Book cover of The Third Mushroom
"Believe in the unexpected" with this hilarious, heartwarming, and much-anticipated sequel to the New York Times bestseller The Fourteenth Goldfish!

Ellie's grandpa Melvin is a world-renowned scientist . . . in the body of a fourteen-year-old boy. His feet stink, and he eats everything in the refrigerator--and Ellie is so…