3684 unique used only once, out of 7746 unique total, and overall 65.4k total words.
Here are the deck stats.
Right, but I would argue that that threshold isn’t as low as one might think.
With this book, when started it I was near the end of WK L31, my kanji knowledge coming almost exclusively from WK and vocab… let’s say 80% from WK (just a rough estimate).
IIRC, JPDB was showing ~45% knowledge with ~80% coverage - and it still was a struggle.
Had I started it at WK L15-20 it would have been far tougher.
Had I started it at WK L52 it would have been easier, not sure by how much but in it I saw a lot of kanji and vocab from higher WK levels than mine.
There were (and there will be in each book) common enough words that simply cannot be used every other page, unless you want stilted language.
Maybe it’s just a me thing, but if I don’t see a word often in the early stages, it does not stick. If you’re the kind that doesn’t need incessant repetion… that’s awesome! 
While reading that book and any other, besides what I get from WK, I’ll also select 3-4-5 from every daily reading session (10-15 pages) to add to SRS because I feel they’re important enough for me to retain them… for various reasons.
I wouldn’t check to see if they are or not in the next book - doing that with the book above for each of the 3-4k new-to-me words would have been… exhausting.
That’s only in the later stages, after it has shown you them a few times at much shorter intervals 