The letters appear raised because of a phenomenon known as refraction. When light travels from one medium to another, it bends (either towards or away from the normal). This happens because different media have different densities and thus, the speed of light is different in each one of them. This causes the light to slow down or speed up, depending on the medium. Note that refraction takes place at the interface of two media, air and glass in this case. Also note that refraction only takes place when the light is incident obliquely to the medium and not along a normal drawn to it.
Thus, when we look at letters printed on a piece of paper through a glass slab obliquely, the light rays bend at air-glass interface and the letters appear raised to us. We can also think about it by considering two light rays from the printed letters. These rays would be refracted on coming out of glass slab (and into air). If these rays were to be extended backwards, they will meet at a slightly raised position, giving us the illusion that letters are raised.
Similar observations are there when we look at the bottom of a swimming pool. It appears raised. A pencil dipped partially in a glass of water appears displaced at the water-air interface.
Hope this helps.