This was probably my favourite book when I was little. I can't imagine how many times I've read it- for me, the perfect blend of reality and fantasy. It's also very funny and neatly written- by J.B.S. Haldane, a leading authority on genetics in his time, and a very famous scientist indeed. There are 6 stories in the book- A Meal With A Magician, A Day in the Life of a Magician, Mr Leakey's Party, Rats, The Snake With the Golden Teeth, My Magic Collar Stud. If you buy any of the later editions, it'll be illustrated by Quentin Blake.
The whole tone of the writing is confiding and breezy, with the author taking the craziest things perfectly in stride- a very calm, English attitude, and the settings are slightly quaint-seeming now, but again, very English and of their time. It nails perfectly the knack of referring to things that may be over the readers head without confusing them, so although there are some allusions in there you might not understand until you're older, it never spoils the story. It's also very short, which is a shame, as to my knowledge it's the only fiction Mr Haldane ever wrote. A must-read, though.