When creating soft-cover books using the Book module in Lightroom 5, you need to have a certain thickness to the book before you can add text to the spine. The limit differs based on the paper type:
– Premium Lustre paper – 84 pages
-Premium Matte paper – 84 pages
-ProLine Uncoated paper – 70 pages
-ProLine Pearl Photo paper – 70 pages
-Standard paper – 114 pages
For NON-softcover books (Hardcover Dust Jacket and Hardcover Image Wrap), you can have spine text even with a 1 page book. So you’ll always be able to input spine text on a hardcover book.