A missing dust cover can cut the value of a collectible book in half or more.
Similarly, a damaged dust cover, or a library dust cover with all its adhesives and plastic, can have a deleterious impact on book value. And dust covers separate from their books are very difficult to source, especially for collectible books.
It is an attractive idea to potentially remake or replace the book's original cover. Speaking from experience it can be a bit of a downer to look at a shelf and know the book is incomplete. Completism is a harsh mistress.
From a book seller's perspective, reproducing a dust cover is something that either should not be done, or disclosed clearly. For most people it would be impossible to pass a reproduction to someone with a studied eye, and for the people that could make an undetectable reproduction, the cost is likely prohibitive -- aged glossy stock, vintage printing equipment, access to original art, etc.
From a collector's perspective though, for someone building a bookshelf and not interested in reselling, replacing a dust cover can be very rewarding.
Quality color printing equipment is accessible and relatively easy to use. The biggest factors in choosing your printer is just the limits on the paper formats. You need to be able to print at least as large as the shorter edge of the dust cover. So for a dust cover that is 12 inches by 8 inches, you need to be able to print edge-to-edge on 8 inches.
The paper stock can be tricky, as it is generally thin, glossy, and easy to smudge with inkjet printing. But if you use somewhere around 80 lb satin stock or better, you can achieve creditable results. We're not trying to fool buyers, we just want the bookshelf to look complete.
Of course, you can go all the way and either buy old stock paper, or damage the paper you purchase, to achieve the effect of an original dust cover. It can be fun, but it is time consuming. But when it's $500 difference between a book with or without a dust jacket on it, a budget can dictate the path to take.
Finding the original art is your next obstacle. If, like I do, you collect multiple copies of the same books, it can be very straightforward. Scan the one you have, at at least 600dpi. Measure it physically to be sure you reprint it in the exact same size, and you'e off.
But if you don't have the original, it's time to hit the image search. Amazon often has very large images of covers that can be used for a portion or as a check, but most of the images without watermarks are limited to 500 pixels on the longest edge. Simply not good enought for a good result.
You need at least 150dpi images to use, and if you can't find it all in one, you can often find an image of the spine, of the front, and of the back separately and stitch them together. You can use any image editor to do it, just be sure to adjust your colors enough to make them all roughly the same palette or at least close to what the real cover looks like.
Obviously the smaller the book the easier the job. Printing on paper larger than 8 inches on a side requires larger, more expensive printers, and the paper is less available. And smaller dust jackets just have less room to show mistakes or imperfections.
If all else fails, you can improvise. You're not selling the book, you're clothing it. You can use the cover art of the paperback, or you can just create your own art to cover the jacket. Sometimes that might even be preferable, if you're neurotic enough to store your dust jackets separately to avoid damage. You can create your own look to adorn all of your favorite books.
At this point you have an image, at least 150dpi, ready to print on some kind of glossy stock. Check and double check the actual output dimensions. Your printer likely can't print to the absolute edge, so you need to know the maximum size it can, and you need to use something other than scissors to cut it after. For me, it's just easier to pay for a color printer at Kinko's. It doesn't smear, they have a number of paper size drawers to work with, and they have those nice razor cutters to finish the job properly.
If you've gotten everything right, and you're happy with the way it looks, it is time to fit the jacket to the book. This is easy to get too tight, or misaligned, as the paper is flat with no bends in it. You need to lay the book and the sheet against a straight edge of some sort ot be sure they stay aligned. And do not fold the jacket, just bend it enough to keep it in place as you carefully close the book.
You can practice all of this with blank paper and cheap books, I would recommend that you do. Once you get the feel, it is straightforward.
A replacement dust jack can be just the thing to complete your collection of an author, or to standardize the look of your bookshelves, or even sections within your collection. It is also a lovely gift when giving a book, you can create a custom dust jacket to make the gift that much more personal.
Whatever your reason, custom or reproduction dust jackets are a lot of fun to make and can look remarkably good. Just don't be tempted to sell them to increase the price on your collectible books. Not everyone will notice, but eventually, someone will.