Morrison and Boyd §§ 4.7 - 4.16 When we have four different substituents in tetrahedral methane, it is possible to construct isomers that are not identical, but mirror images of each other:
In textbooks the structures are normally drawn with horizontal bonds pointing towards you, and the vertical bonds pointing away. Arrange the structures in such a way that they form left and right mirror images. Then try to rotate one structure so as to become identical with the other. If you succeed, you didn't look properly! You can get any two atoms in the same position, and then the other two are interchanged. Try it!