If all you did was add an SD card, but you didn't tell the phone to actually use it, it's probably still trying to load your books to the phone's internal storage.
If you go into Apps - e.g., find Kindle app in the list - you can see if it gives you the option to move the books and even the program itself to the SD card. Or if you open the program you're using to read books, you might check the app's settings to see if it gives you the option to move them to the SD card.
I'm probably not explaining this very well. I tend to switch as much to SD card as I can, then I forget about it. So it usually takes me a while to find the options when I need to go back and revise my settings.
I also had a Samsung phone that removed the ability to move apps/files to the SD card via the Apps menu. Probably why I haven't bought another Samsung device.