Translation:Will you read this book?
You know by context. If your teacher hands you your next book assignment but knows you're considering skipping your homework tonight and asks, paqvam DalaD'a'? you can be fairly certain they're asking about your intentions for the future rather than anything else.
Without context, as an isolated sentence, there is no way to know when the sentence takes place. Klingon does not mark its verbs for tense, or when an action happens.
By the way, Did you read this book is most likely asking about whether you completed an activity. As a completed action, it needs a completed suffix: -pu' or -ta'. If, instead, it's asking something like whether this is the book your class had to read last week, not whether you completed reading it, then no completed suffix is warranted. Be careful when translating the past tense in English: determine whether you're talking about a completed activity or not.
The English use of "reading" indicates an ongoing activity which would match with the Klingon use of -taH for an ongoing activity. "Do you read this book?" is a present tense version that is accepted on this exercise. The problem is that English doesn't really use that simple present tense for things that are actually happening at the present, instead, English insists on saying that it is currently ongoing as a description of a present action. In some situations where one might say, "Are you reading this book?" it would probably be appropriate to use the simple unmarked Klingon paqvam DalaD'a'? However, to make sure users indentify -taH with an ongoing nature of an activity, we have decided that in this course, we do not generally accept the English progressive tenses unless the Klingon has -taH and require -taH to match with the English progressive tenses.
To supplement what jdmcowan says...
In English, Are you reading this book? is in the present progressive tense. English splits verbs into two main categories of action classes: states (where nothing happens, like knowing the answer) and events (where something happens, like reading the book). In the present tense, states are expressed in the simple present tense (I know the answer) and events are expressed in the present progressive tense (I am reading the book now). You wouldn't say things the other way around (I am knowing the answer; I read the book now). In English, you use a progressive tense whether or not you actually want to express the idea that an action is progressing.
Klingon does not observe this state/event distinction at all. (It has its own verb distinction of quality vs. action, but that's a different discussion.) Klingon -taH is not the same thing as English progressive tenses. When you use -taH (or -lI'), you are expressing the idea that the action is LITERALLY ongoing or progressing.
So when a student of Klingon wants to translate I am reading the book, they may have a problem: they may not be aware that the English sentence is in the present progressive tense, and they may not be aware that English requires this because the verb describes an event instead of a state. Therefore, it may not be clear to them whether the sentence is better translated as paq vIlaD or paq vIlaDtaH.
As jdmcowan has said, this Klingon course generally requires that English present progressive tense be translated into Klingon with -taH or -lI' merely as a method of simplifying teaching. This neglects what the sentence you're translating actually means in favor of a simple one-to-one correspondence of aspect to tense.
So the real answer to the question, can you translate paqvam DalaD'a' as Are you reading this book? is yes, depending on what you mean by it, but you may find that Duolingo doesn't accept that translation anyway.