sync says items not copied

Why are things that should be so easy so hard?

I downloaded books to read on my iMAC. I click on sync all because I want to sync all. Yet, that doesn't happen.

This error message comes up sometimes for books, sometimes for music...why can't everything just sync???

Does anyone know how to get these books over to my other devices? And, if possible, can anyone tell me how to avoid this crap in the future?

I was doing the corded sync to my iPhone and got this...

Error Message:

Some of the item in the iTunes library, including "NIV Study Bible (Red Letter Edition)", were not copied to the iPHONE "Tarik Grabowski iPHONE" because they could not be found. (My words-->WHY NOT? THE *#(N THING SYNCED TO 1/2 OF MY OTHER DEVICES)?

Error Cont.:

For a list of items that could not be copied, click the triangle below.

Then 3 other items are missing.

ALL I WANT IS FOR MY DEVICES TO SYNC THE MUSIC WHERE I PUT IT AND THE BOOKS WHERE I PUT THEM. THIS HAS FRUSTRATED ME SO MUCH BECAUSE I BUY ITEMS FROM APPLE, BOOKS OR MUSIC, AND IT'S ALWAYS A PAIN TO GET STUFF TO MATCH.

I OWN AN IPHONE/IPAD MINI/IPAD AIR/IMAC & LAPTOP. For all that money spent, things should be so much more easier to use. These aren't even major things.

THANKS FOR LISTENING AND ANYONE THAT CAN HELP, PLEASE DO!!!! I JUST WANT EVERYTHING TO MATCH.