Mary Pope Osborne - Good Morning, Gorillas

Book 26 in the Magic Tree House series (2002)
A novel by Mary Pope Osborne
The Magic Tree House whisks Jack and Annie off to the mountains of Africa. There they run into a huge mountain gorilla! At first they don't  know whether they should shake hands or turn tail. But the ominous-looking creature turns out to be surprisingly gentle. Not only that, the gorilla may be able to help them learn their next bit of magic, which Morgan has challenged them to do.