You use a bookshelf as a book counter. Duh. What are you gonna count them one by one? Okay, well maybe, but you should count them while they're nice and organized and on shelves. Otherwise counting is going to be a huge pain.
While you were counting books using this book counter method, you got a bit tired and sat (さつ) down. But lucky for you, your books counter is a bookshelf, so just because you sat down doesn't mean you have to stop counting books.
Currently there are no similar looking kanji which exist in the WaniKani learning system