iPad Sync interrupted as "USB connection lost" - possibly due to full memory

I have added some books to my Kindle App and when I have next tried to Sync with my iMac (late 2009) the message passes rapidly from step 1 Copying Diagnostic Information to step 4 transferring photos and then produces the message that "The iPad could not be synced because the connection to the iPad was lost". The iTunes App reports that the memory is almost full but the Ipad Storage and iCloud information shows that 21.1GB Used and 5.3 Available. I have tried deleting books from my Kindle account and with advice from Amazon I have deleted some books on the iPad. iTunes still the memory to be almost full.

What can I do? Many thanks. Hugh