we can say " Some idiot driver crashed into the back of me."
but why we can not say " If you’re looking for some book to read, I can recommend ‘Animal Farm’."
we say "a book," the question is why we use some with countable singular nouns somethimes , and sometimes , we do not use it with singular countable nouns ?
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).
I'm not sure I'd agree with OP that we can't say If you’re looking for some book to read... But I will admit that in most contexts it would more likely be If you're just looking for some book to read...
Other answers focusing on the literal distinction based on a specific [but unknown] driver are missing the point of this idiomatic usage. In practice, for contexts like OP's above, you'll very often see either the word just (as in Q:Who told you that? A:Just some guy in the pub), and/or a pejorative adjective (as in Q:Who runs Elbonia? A:Some tinpot dictator).
In essence, this (primarily spoken, colloquial) use of some X implies a randomly-selected instance of X, whose precise identity is of no consequence (all that matters is that it / he is an X, which is "bad").
Thus the implication of looking for some book to read is that in the speaker's opinion the addressee isn't interested in the finer points of literature - just about any book would do, even if it's poor quality.
A driver crashed into me - Articles are more of a signal to the reader/listener than anything for the writer/speaker. Whoever is saying this merely expects you not to know which driver he/she is talking about. The writer/speaker may or may not know, but probably does (otherwise they'd use some).
Some driver crashed into me - This means even the writer/speaker doesn't know which driver actually crashed into him/her.
"He is in some sort of trouble", "There is some reason to support this"“She won’t be here for dinner. She’s spending time with some friend of hers.”
So yes, some is sometimes used with singular count nouns.
It has a quite different implication than when “some” is used as a plural indefinite article (where it doesn’t imply anything.)
It often suggests some unknown person, some undefined trouble, some not so-clear reason.
If I said, She’s not joining us. She met some guy that she’s spending all her time with. it has a very negative connotation. The other two cases not so much negative as vague or unknown.
My daughter points out that “some” can also be used as a kind of superlative, as in Charlotte’s Web: “Some pig!” That’s some car he’s driving! But it can also imply sarcasm: Wow, that’s some dress she’s wearing. (meaning, it’s so bad I don’t even know where to begin.)
I think "unspecified" is the thought that connects all these uses. Other examples or explanations welcomed! (Based on a question asked on Quora)
