In your example sentences, some is being used as a determiner to:
refer to a particular person or thing without stating exactly which one - Cambridge Dictionary definition C1
More than this, it is usually used when the identity of the person or thing is unknown, or if it is known and the speaker deliberately wants to be vague, but there is a specific person or thing they are referring to. It is only really used like this in an informal context, or in speech. I wouldn't say that its usage is limited to certain words, more to certain contexts.
In the first example, it is not just any "idiot driver", it is a specific "idiot driver", but one who is unknown to the speaker.
In your second example sentence, they are looking for any book to read, not a specific, unknown book, so "a" would be more appropriate. I wouldn't say that using "some" book is wrong, but it either makes it sound like you've missed off the a plural "s" (some books), or perhaps that you are criticising them for looking for a book to read.
You could, for example, say, when asked what you're reading, "I'm just reading some book." (I can't remember/it doesn't matter what I'm reading) or "I'm reading some Harry Potter book." (there are a few Harry Potter books, I'm reading one of them, I can't remember/it doesn't matter which one).