"My husband is reading a book."
Translation:Kocam kitap okuyor.
Actually, to talk about "my husband" or "my wife", it is better to use "eşim", meaning "my spouse". This way, it is more polite.
However, if you compare "kocam" and "benim kocam"; the latter sounds more like you are making a comparison. Like when two people are talking about their husbands and compare them :)
Would it also be right to translate this as "my husband is reading books" (as we've been taught to translate, say, "kocam elma yer")? Putting kitap in the singular accusative case would of course make it definite ("kocam kitabı okuyor" = "my husband is reading the book"), but is there anyway to make it clear that one is referring to a single, but indefinite, book - e.g "kocam bir kitabı okuyor", or is that wrong?
If you want to make it a single indefinite, you can say either "kocam kitap okuyor" or "kocam bir kitap okuyor" both are indefinite. They are almost the same. The only difference that I can think of is that the former might be plural like you said, or it can be any book, while the latter is pointing out that the husband is reading "a single" book unknown to the speaker.