Some books are borrowed more than others. You're more likely to earn lots of PLR from a racy romance than a handbook on cello maintenance, for instance. People borrow books for different reasons. They borrow books that they don't want to buy - either because they don't want to keep them after reading them, or because they can't afford them in the first place.
Picture books get a lot of loans because many people can't afford to buy lots of books for their children. Many who could afford them think (wrongly) that the book will take 20 minutes to read and so it's poor value for £8.99 - forgetting they will read the book again and again. Genre fiction earns a lot because readers consider it disposable. They read a romance/western/sci-fi book and will never look at it again. They might get through several a week. That's an expensive habit if you pay for each book, so libraries are attractive suppliers. (Loans of genre fiction might soon be hit by Kindle use - we'll see.) Non-fiction loans seem to be down, probably because people are finding information online that they would previously have looked for in a book.
My PLR goes up, year on year, because I write a lot of books. But libraries are buying fewer books. That means that having a backlist already in the libraries is important, too. But old books fall apart and are thrown away, not to be replaced, so very old titles don't earn PLR.
So - what's borrowed? No detailed breakdown for this year is available yet (though the annual report from PLR is), so I took a look at my own titles to see what's earning most.
Of 150 registered titles, half the PLR comes from the top four titles - they are all children's fiction. Of the top 10 titles, 9 are fiction. More of my titles are non-fiction than fiction, so that's a very significant result. The non-fiction title in the top ten is a glossy, very visual book on volcanoes published by Dorling Kindersley. Of the 9 fiction, all except one are published by a very large publisher, and all published since 2007. The books with titles most likely to appeal to children (in my view) did not score higher than those with less intriguing titles. (The official info that has been released so far shows loans of children's fiction up slightly, loans of adult books slightly down.)
My top-earning title clocked up two and a half times as many loans in the UK as sales worldwide. It's called Too Dirty and is published by Hachette. All the top-selling fiction titles were for children aged 5-6. Even though the PLR is split 50:50 with the illustrator on these titles, the books generate a third of their advance each year in PLR.
The lesson from all this is that over its lifetime a book may earn more in PLR and ALCS payments than in fee/advance/royalties so we should be thinking about library-appeal as well as bookshop-appeal. Of course, the publisher is less interested in library-appeal so this has to be a covert agenda!
Which are your most-borrowed titles? Can we crowd-source enough info to put together a profile of the perfect book to write to get loads of PLR? Or at least, you might be able to maximise PLR-potential on your next book...
[PS - No, this is not an entirely serious proposal]