You can use だ and か together. I think だか is short for であるか. For native speakers, であるか sounds like a formal speech. So we often use だか.
For example, of course you can say
どの人が李先生か知っていますか。
but you can also say
どの人が李先生だか知っていますか。
so your textbook is wrong, and
どの人が李先生であるか知っていますか。
is also Okay, but this is a little bit formal. (maybe young people don't say so.)
Another example, you can say both of
この人だれか分かりますか?
この人だれだか分かりますか?
There's no difference between them. However
この人だれであるか分かりますか?
is strange. Because in the latter part, it uses であるか so this sentence should be formal, but in the former, は or が is omitted. So
この人がだれであるか分かりますか?
この人はだれであるか分かりますか?
is Okay, but I don't usually say so to my friends.
Now, turn back to the main topic. As far as I have mentioned, it seems there is no difference between だ and だか, but unfortunately it is not true. Actually, you can say
これは本か。
but you cannot say
これは本だか。
This is difficult to explain, but probably because many people feel a sentence doesn't end by だか. Actually, if I heard the latter, I would think "What is Hondaka?". To sum up, these are basically the same but don't end a sentence with だか.