When I saw the hiding places contest I thought of many solutions. My favourite ever since I was a boy was the book with a secret compartment.
I was going to make one when I saw that there are several such entries.
I read each entry and saw that no one had created one like the one I made many years ago.
You see this approach allows the book to be flipped through whilst still keeping the secret compartment out of sight.
Thus is a TRULY secret book.

Step 1: Select a Great Page Turner

You will need a good thick book. Hard back is best but a paperback will also work.
(Do ask the owner of the book for permission and do not use a library book....or a kindle)
I thought that this one had a nice appropriate title too.

Step 2: Don't Start at the Beginning

Turn a few pages into the book and round about the first chapter start to stick down the extreme left hand side. I used a glue stick which sticks well and does not crinkle the paper.
This is a tedious task as you now need to repeat this for every page until you reach the depth that you want.

Step 3: An Overnight Success

After checking that you have glued all the pages needed, it is best to clamp it up and leave it overnight.

Step 4: The Cutting Edge

Using a guide you can next begin to cut through all the pages to produce a long narrow but deep hole. Take your time and always cut away from fingers.

Step 5: The Plot Deepens

I found that after a while my super sharp knife could not go deeper due to its shape. I switched to a longer knife which I sharpened up before use.

Step 6: The End Is Hard

I found that the short ends were the hardest to cut. The best method I found was to use a sharp pen knife to whittle away at the ends.

Step 7: A Magnetic Plot

Whilst having the secret section close to the spine hides the secret, it is no use if the cavity can be seen at the start of the book.
This is why the cutting started later.
A row of small but powerful magnets are pasted between the first two pages of the hole.
Next on the page just before the hole another row of magnets is placed. You will have no problem aligning them since they will snap into place as they are attracted by the row below.
Paste down the page before that and you have a secret flap that can be pulled back to gain access to the storage cavity.

Step 8: Nothing to See Here

To a casual observer pages can be turned without noticing the secret.

Step 9: The Secret Within

Lifting back the magnetic pages reveals the storage pocket. Deep enough to to hide many treasures.

Step 10: Oodles of Storage

Hide memory cards and there is enough space for terabytes of storage.

Step 11: Thumbs Up

If you prefer to stuff it with thumb drives then again it could hold gigabytes of data.
Or
Millions in money
Or medals of honour.

Step 12: Secreted Diamonds

More than enough space to safely hide jewels worth millions.
Note to nefarious fellons; the items shown are not worth millions.

Step 13: A Plot Twist

Flick through your new super secret book to make sure all the pages are stuck and marvel at not seeing any evidence of a cavity.
Note: in the picture you can just see the hole top left...so a little more glue and there's no more clue.

Step 14: A Happy Ending

And there you have it.
A book with a secret that is less likely to be discovered.
A great hiding place for valuables or 'personal' items.
Of course no member of the INSTRUCTABLES community would have anything illegal to hide.

Enjoy.