No, I’m not kidding. ebooks have a weight of their own. Since there are electrons involved to store the information, and most ebook readers use flash memory it is actually easy to calculate the weight an ebook has, although it is difficult to actually weigh it.
Michael Stevens has a TedEd video with the answer. Actually he has the answer for the weight of a video, but since an average ebook has the 1/100 file size of a video we can calculate it to be 10^-21 grams or 0,000000000000000000001 grams.
You can also say that it weighs “nothing” and be very accurate since our best industrial scales can understand weight changes of just 0.0000001 grams.