What's the difference between book and slip match?

Book Match Veneer
Book match is the most common veneer leaf assembly. The consecutive veneer leaves are “opened,” like a book, creating a mirror image of the previous leaf as they’re spliced together. This matching gives an aesthetically pleasing continuity of the wood grain. Since one side of each leaf can reflect light or accept stain different than the other side, there can be a light/dark contrast. This phenomena is called “Barber Pole.”

“Barber Pole”
This condition starts at a cellular level in the wood and is caused by how light is absorbed and reflected off the face of the veneer.
This striped effect is not considered a defect, but can be minimized by specifying slip match (described below). 
Not every book matched door will have the barber pole effect. 
It is, however, particularly common in rift cut red & white oak. It can also be more pronounced when darker stains are used.

Slip Match Veneer
Slip match is the second most popular veneer assembly. The consecutive veneer leaves are “slipped” in sequence without flipping them. All the same face sides are then exposed. This means the grain pattern has a repeating effect as shown in the image here. 
This can result in a sharp color contrast at the splice joints (especially when “natural” faces are specified). 
Slip match is often recommended for quarter-sliced and rift cut – to minimize the barber pole effect