In the family gift exchange I got my 13 year old brother, who is really into spy stuff. In thinking of what I could make him I came across an instructable for a hollowed out book. I had done it before and it worked really well, so the first inspiration for this gift was:
https://www.instructables.com/id/secret-book/

In addition to the book I thought it would be cool to put something inside that was spy related: fake passports, finger print dust, etc. I settled on some kind of invisible ink and a writing utensil. After some investigation, some trial and error, and some inspiration from:
https://www.instructables.com/id/25-Cent-Invisible-InkPen-UV-light-visible/

The following is the steps it took to build a spy hollowed book, with a ordinary (in appearance) highlighter, and a black light to reveal the secret information.

Step 1: Gather the Materials

I spent $10 on the gift, when all was said and done.

Materials:
Book ($1-$3)- Any book from a thrift store which is hardback and has a sturdy spine will do.
Glass candle ($1)- At Wal-Mart for 1 dollar
Black light ($5)- I chose an LED black light on Amazon.
Highlighter- An old highlighter I had lying around.
Felt ($1)- I used one sheet from Wal-Mart
Elmers Glue- I had some at home, but you can find a little thing of it at Wal-Mart for a buck.

Tools:
Paint Brush
Razor Blade
small pan
metal bowl
scissors
containers for ink mixing

Step 2: Hollow the Book

The first step once all of the materials and tools are gathered is to prepare the book. I wrap the first couple pages and the front cover in plastic wrap to protect them from the glue. Next I squirt glue onto a plate and brush it onto the pages of the closed book. People recommend a water glue mixture but I have found I like to use just glue. It dries clear and hard. Try and brush with the grain of the pages, also I usually do 3-5 coats of glue.

Next you take a pencil and draw an outline of where the hollowed portion of the book should be. After outlining where to cut, take a razor blade and cut along the lines. As you cut the shape peel the pages out of the center.

After cutting through 50 or so pages I usually brush the sides of the inside with glue and let it dry before continuing (it just adds a little stability). Continue cutting and peeling out pages until you reach the desired depth.

Finally re-paint the outside pages and inside pages with another coat of glue.

For some books I have place the title page in the bottom, or a picture from the book. In this gift book I measured out the inside edges and cut out felt to fit inside. I glued it with the elmers glue, but I think it would work just as well (and faster) to use hot glue.

Step 3: Make the Ink

I tried a lot of different things with the ink. People suggested liquid laundry soak, highlighter ink, and some kinds of glue. I tried a lot of different mixtures and will explain how to make what worked best for me.

First I took a highlighter (I used a thin one because they are easier to open). I took it apart, being careful not to break anything. I rinsed the tip until it was white, and then took the cartridge and stuck the bottom end in an empty cup of water and ran water from the tap onto the other end in a thin stream. This rinses the ink in the cartridge into the cup of water and dilutes it. I'm not sure the ratio, but about a cup of liquid is in the cup once the cartridge is empty.

Then I took the cup of liquid from rinsing the highlighter out and added 3 drops of liquid laundry detergent. This mixture showed up the best on the paper. The picture doesn't do it justice. Under the black light the whole message shows up very well. To prevent the ink from creating wrinkles in the paper as it dries, I used marker paper (.20 cents a sheet at the local craft store). Just dip the tip of the highlighter into the ink, tap it off and write.

Step 4: Ink Jar

I used a metal bowl and a pan as a double broiler or water bath type idea to melt the wax out of the glass container. I then filled it with ink, stopped the top, and sealed it with the hot wax so it wouldn't leak. All of the components are ready to go into the book and be wrapped.