Two genes might be alleles, or they might not be alleles.
+ + +
No. You either have a bad book (not sure if they exist) or you read wrong.

Perhaps what he meant is that if you have two genes from different loci on the
chromosome, they are not alleles. Alleles are two different genes found in the
same location on different chromosomes, having a different effect on the same
trait, such as a gene for a normal pigment enzyme and a defective gene which
does not create that enzyme. When you breed a plant with the defective gene on
both sets of chromosomes, you get white flowers.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)