Will not download purchased book from iBooks???

i'm having this problem too. I have tried rebooting my ipad but it didn't help. I purchased an iBook but it won't download to my ipad.

I have this problem too.

I Just figured this out. I had downloaded a bunch of books on my iPhone and had the greyed-out issue on my iPad with sync collections enabled. Had tried all the same fixes as indicated in this thread and others without success. What worked was restarting my iPad (this may not be a necessary stage) and searching for the titles that were stuck In the iBooks store. As expected each showed an 'in progress' download button. I was finally able to tap this to stop the (non-existent) download, which hadn't been possible within 'My Books' or the 'Purchased' tab on the store. When I tapped the button briefly jumped to 'Read' then changed to the standard iCloud download button for a previous purchase. Tapping this successfully initiated the download, which then synced into the right collection in my library.

Hope it works for you!