Is Apple Books losing rights to publish a lot of books?

Just recently I’ve noticed that several books (In series of books I’ve been reading and following on Apple Books) suddenly either disappear or are no longer available to buy. This is after I’ve either downloaded a sample and marked them as want to read as soon as they are published. I’ve had at least three from three different series disappear on me as well as another two suddenly not be available for purchase; all after I had downloaded a sample. I’ve checked the ones that disappeared through the internet and all have been released by their authors. What’s going on and is Apple having problems continuing to supply updated and new releases?