I've done some maths to get this thread back on track.
With non-Luck of the sea rod, there is .8% chance of an ebook. That means you will fish 1 ebook every 125 fish.
With Luck of the sea 3, there is 1.9% chance of an ebook. That means you will fish 1 ebook every 53 fish.
There is about a 5% chance that an ebook from fishing will be mending. That means you will fish 1 mending ebook every 50 or so ebooks.
With these statistics, with a LOTS 3 rod, you will catch 1 mending book every 2650 fish.
Assuming the average 17.5 seconds per fish with no-rain fishing chances of 5-30 seconds per fish with Lure 3, it will take you 46,375 seconds, or 773 minutes, or 13 hours of fishing to get one mending book from a Luck of the Sea III, Lure III, Unbreaking III, Mending rod.