Different Mutations Leading to Same Allele?

You are looking at high school biology books. if that. I'd be shocked if a high school text didn't mention blood group genes, for instance.

The simple genetics that Mendel elucidated only applies to a very small number of situations.

In real life, very few situations are explained by the interactions of only two different alleles, one of which is classically dominant to the other, within a single gene.

So sure, you could have many alleles which break a gene, but where the presence of one working allele is enough to be functionally indistinguishable from having two working copies. You could also have slight sequence differences that are functionally identical.

Here's a random example; all the variants ensembl knows about for a random gene. Note that these are only variants which have been found, obviously private individuals and families might have their own variants that are not documented

http://uswest.ensembl.org/Homo_sapiens/Gene/Variation_Gene/Table?db=core;g=ENSG00000280165;r=13:61409685-61415522;t=ENST00000409204

Most of these are in the 3' UnTranslated Region, and likely have no functional significance.