Why can't iPad transfer see my books???

i have a couple hundred epubs loaded into the Books app on my mac. i would like to be able to transfer a handful of them to my iPad whenever i like. so i connect the iPad to my mac, i choose it in the finder, i click on books tab, click the radio button for sync selected books, and... it shows THREE books. every other one is simply not there. there's nothing that i can find in books that makes those three any different from the other 200 or so. i don't particularly care why i can see those – i want to know why i cannot see the others. this is infuriating. i can work around by putting the files in icloud drive and opening them that way, but it's inelegant and a pain in the part of my body that contacts the chair i'm sitting on. someone please help!