Yes, Dan Brown would have a higher probability of more useful words than HP, but whatever keeps a person‘s interest better is what they should be reading. I remembering reading Da Vinci Code in Spanish when I only had like 12k words and it felt too easy for what I was going for (I usually take on more unknown words).
Did you import the books into lingQ somehow? and did you start these books as a beginner and just pushed through the difficulty or were you already a little experienced by then?
Personally I find challenges quite useless and prefer streak. If I would lose my streak I would be devastated. 
As said before it’s usually people that already speak the language.
Started with the first HP book when i had around 20k knowns in russian (heavily inflected language so for other less inflected ones you probably don’t need as many)
I remember having around 25-30% unknowns at the beginning with the first book, now it’s rare to have anything above 15% and sometimes forget i’m even learning a new language.
I do have to admit it wasn’t easy as first, but gradually got easier as i progressed through the book.
I agree. I just joined but I have been study Chinese for 4 years already. Still creating lots of lingq but I ended up in the number one position on the 90 day challenges I did. Sorry to the folks I accidentally crushed.
Last December I started a 30 day challenge in Spanish to refresh my speaking skills in that language which had been eclipsed by my intensive Russian studying. I deliberately spent most of the time LISTENING because I knew that would be the most effective to reach my goal of improving my SPEAKING. (It was.) I did this challenge FOR MYSELF and in a way that benefited ME. Yes, the number of words that LingQ added as known each day was high because they primarily were words that I already knew when reading. I don’t apologize for that. In fact, that I still understood these words when only listening was itself quite gratifying to me. In addition, my comprehension of spoken Spanish definitely improved during the course of the challenge. I do not think that I shouldn’t have engaged in the challenge to begin with.
One uses the “challenges” in whatever ways one finds effective for one’s own language learning. I personally didn’t care about my relative standing with others in the challenge with respect to words learned, the number of lingqs made, words read, or time spent listening. I did it for MYSELF as indeed this is how I use all LingQ functions and statistics.
I agree that those who game the system – cheat – are to be scorned. There are myriad ways to do inflate one’s statistics: : e.g., clicking on virtually every word (including every conjugated form of a verb or declension of a noun and adjective in inflected languages), paging through lessons or letting the audio play without actually reading, listening or understanding the content, etc… Who but a very sad and insecure person would spend so much time inflating their statistics to gain some position among strangers? To what end? What does such behavior say about how unrewarding the rest of their life is that anyone would bother with such a charade about their activities and accomplishments?
I recommend ignoring these cheaters. They want attention. Don’t indulge them.
Focus on yourself. The challenge ideally is a way to spur you to do more than you would normally or, as in my own case with Spanish, of revealing something new about which activities (listening) are more effective. The great benefit of LingQ is that it facilitates independent language learning. Any challenge is ultimately about yourself.
I “learned” sometimes more than 1000 words per day, but I knew these words already. 50.000 words sounds great, but one word has often more than 3 forms. e.g: work, worked, works… and so on.
My writing and grammar skills are still not good, but I use LINGQ mainly for reading practise to understand newspaper ond TV. Sometimes I read more than 6 hours per day.
Effective I learn 10-15 words per day, not more.