Babies must be changed. Some folks like to change 'em around hip height. To be sure: you can buy a fancy changing table on which to bring about change. Or you can change your bookshelf into a stand for a tray on which change can naturally occur. And when the baby is done with changing? Discard or donate your changing tray, and keep your bookshelf.
Step 1: Measure Your Bookshelf, and Procure Your Materials.
Your changing tray will be custom made to fit on top of your bookshelf. So you must measure its dimensions to know how much wood to get.
Our bookshelf is 31 x 15 1/2 inches. So we procured some woods with which to build a tray that fits on it.
Our base is made of 1/2 think pine board, cut to a 32" x 16" rectangle - a wee bit bigger than the top of our bookshelf, so as to make for a not-too-tight fit.
The walls of our tray are made from 1/2" by 6" pine planks. Two six foot long pieces were more than enough.
We used some handy dandy corner braces that we got from the Home Despot (see pic), along with some wood glue and some longer skinny wood screws suitable for screwing the planks in at the corners.
Even though we are prejudiced against hippies, we still act like them sometimes. So we use natural untreated woods.
Step 2: Cut and Sand the Wood
You need to cut the bottom of the tray to a size at least 1/2" wider and longer than the bookcase it will fit upon. Then you need to cut the side planks - 2 long side ones, and 2 short end ones. These will surround the bottom. Sand the edges of the sides so they look nice.
If your wife is pregnant, consider exploiting her nesting instinct energy, and make her do all the sanding.
Step 3: Prop the Floor of the Tray Up
The base of the tray needs to be above the bottoms of the side pieces by about 1 1/2". This way the tray will fit on top of the bookshelf, and the sides will keep it from sliding off.
So before you attach the sides to the bottom of the tray, put some scrap would below it, so it sits about 1 1/2" above your work table.
Step 4: Glue and Screw on the Sides
Put some glue on the edge of your slightly elevated tray floor. Then put two of your side planks against the floor to form a corner. Glue the side planks where they meet as well.
Now screw the handy dandy corner brace to hold it all together. If your floorboard is splitty, consider drilling a small hole first so the screws don't make it split.
Now repeat with the other two side planks.
Step 5: A Bit More Screwing
Sure, corner braces are cool, but we want extra strength, for strenuous changing.
Drill holes in the side walls, and put two longish (~2 inches or so) screws in each corner.
You are almost done!
Step 6: Put Something on the Bottom to Keep From Scratching the Bookcase
We ended up having a lot of play, since we overshot the extra 1/4"-larger-than-top-of-bookshelf dimensions I now recommend. So I cut a piece of yoga mat into a rectangle, and screwed it onto the bottom of the tray floor. That keeps it from sliding around, and avoids scratching up the bookcase on which it rides.
As you can see, I also put an extra bead of wood glue in the seam between the floor and the walls. It can't hurt.
Step 7: That's It!
Now put it on your bookshelf, and put a cushion or some blankets in it, and voila!