One of my textbook says: だ and か can not be used together, like:

Example: これは本か。(Is this a book?)

You can NOT say: これは本だか。

The textbook also says:

疑問詞（ぎもんし）（どれ、どちら、どなた、どこ、だれ、いつ etc) When used in a sentence with か, you should
not use だ.

Another example: どの人が李先生か知っていますか。

You can NOT say: どの人が李先生だか知っていますか。

However, when I surf the Japanese websites, I noticed sentences like this:

この人だれ **だ** か分かりますか？ Here, だ and か is used together.

Isn't it supposed to be この人だれか分かりますか？

Does that mean the textbook is not accurate?
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grammar
だ　and か, Can they be used together?
You can write だか in an embedded clause (usually containing a question word
such as どの or だれ) which is a complement of a verb such as 分かる. In a main
clause, you must omit だ before か.

So I think you **can** say:

> どの人が李先生だか知っていますか？
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だか is used with question words all the time by us native speakers. However,
you need to know that it is used almost exclusively in informal speech as the
more formal forms are であるか and であるのか.

Examples: (Note that I will make the sentences informal on purpose so that だか
fits in naturally.)

「何が何だかさっぱりわかんねえ。」= "Dunno what the heck's going on."

「これ、どこのワインだかわかる？」= "Can you tell where this wine comes from?"

「なぜだか教えてくれ！」= "Do tell me why!"
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It sounds like you are learning and still in the relatively early stages in
which case your text book is introducing you to new grammar step by step.

If you look a few chapters ahead you will probably be introduced to more
complex sentences such as:

> いくつあるか知りません | I don't know how many there are.
>
> いくらだか知りません | I don't know how much it is.
>
> だれだったか忘れました。| I forgot who it was.

These came up in chapter 26 of one my first text books (p338 of an
Introduction to Modern Japanese, which is quite old).

As you can see from the other answers, these are first steps to more complex
uses.

I would suggest you take it step by step (but at a pace you feel comfortable
with!). As you come across new more complex but useful phrases that you can
remember, learn them as set-phrases. You probably know a few already
(どういたしまして？）. When you get to study the grammar in your studies, it will be a
bit easier because you will have passively built up a feel for it. It is quite
easy to study lots of new grammatical rules and feel you understand them but
harder to adopt them all at once (or at least that was my experience).
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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.
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