If you love books, and you love living with books, then those books need somewhere to go. The problem is that most bookcases are really, really boring. But fortunately there are a few DIY ways you can make your bookcases just as interesting as the tomes you place on them — without spending a ton of cash.
Above: From Livet Hemma comes this IKEA BILLY bookcase with an exciting pattern (inspired by the fanned pages of an open book) on the inside. While this particular design is actually a limited-edition version of the BILLY with the pattern printed on the back and shelves, you could re-create the look by covering the back panel of a bookcase with wallpaper. (You can find a DIY for that here.)
Although it may look like wallpaper, this bookcase from Brittany Makes is actually covered with paper with a hand-stenciled print. You can find the DIY here. Or could take the approach of Diane from In My Own Style, who cut pieces of cardboard to fit the back of the shelves of her bookcase, and then covered them in fabric. This way, it's easy to remove the panels and change the fabric when the mood strikes.
This image, from Lonny, is actually of a high-end store, but it's a lovely idea that you could easily adopt for a bookcase. Just have colorful plexiglass cut to the size of the shelves in your bookcase, and pop it in. (Alternatively, you could have clear plexiglass cut to fit your shelves, and then put patterned paper under the glass.)
Here's another idea, also from Lonny: paint just the back panel of a bookcase for a little pop of color.
Or for maximum impact, paint the entire inside of the bookcase, including the edges of the shelves, as with this piece from LaRabotine on Etsy.
Or you could take a cue from The Land of Nod and replicate this ombre bookshelf. Painting just the shelves seems pretty doable, but if you're not much of a DIYer, you can also just buy this one for $449.
And finally, from A Cup of Jo, a painting idea that might be a bit time consuming, but very rewarding: paint the inside of each level of shelves in a different brilliant color. Or you could create an ombre effect, with different shades of color moving from dark at the bottom to light at the top (or vice versa) — your imagination is the only limit.