Esio Trot by Roald Dahl
Mr. Hoppy is in love with Mrs. Silver, but her heart belongs to Alfie, her pet tortoise. Mr. Hoppy is too shy to approach Mrs. Silver, until one day he comes up with a brilliant idea to win her heart. If Mr. Hoppy's plan works, Mrs. Silver will certainly fall in love with him. But it's going to take one hundred and forty tortoises, an ancient spell, and a little bit of magic.