I found this on a apple support site. I had to try it a few times to get the timing right, but it worked for me
—>>> Thanks to iuser01, sorry no idea how to link their post directly
If you have iBooks open – close them.
Open them again and while little circle on top of iBooks left side (next to Wi-Fi bars) is spinning, (which indicates it’s loading the content) press Feature tab.
It should force the content of Feature tab to start loading.
Once it’s loaded all other tabs start working, but only until you go to “My books” tab.
When you’re in My books and everything is loaded and you press the Feature or any other tab it will not work/load.
So, looks like load of those tabs in iBooks store must be “forced” and only way to force the load is to press the Feature tab while that circle is spinning.