Translation:The Quran is the holy book of the Muslims.
That doesn't mean anything. You simply cannot say like that because it doesn't have any meaning.
You have to put an "a" there because you refer to them. Like "a lor"= "their".
The same is this case "a musulmanilor", because it's their, not just a simply book. So you have to say "a" to specify that.
I hope I made myself understood :)