I have worked out a simple way to put a single picture in the background on a two-page spread!

Question

Please remember, if you have been working on your book, click on Create Saved Book to save your book BEFORE you do this.  You can lose all your work up until the point where you create a saved book.  After that, it appears on the left menu bar under Collections with whatever name you saved it under.  Your book will most likely not appear when you click back on the Book module after leaving it.  Don't panic - it hasn't disappeared.  If you clicked Create Saved Book to begin with - it's there under Collections.
So here's the exciting bit (well the solution anyway):
Click on the picture you want to use as your two-page spread
STEP 1 - In the Library module
select your image (e.g. X.jpg) and right click on "Export..."
send a copy of this picture to "Same folder as original photo" (DON'T overwrite! remember to use a unique name, such as X_1.jpg)
then repeat to create another copy (use another unique name, such as X_2.jpg)
STEP 2 - In the Develop module, take X_1 and:
click on Crop Overlay (the dotted rectangle at the top of the right menu, just below the histogram)
select a region that is 50% of the width of your original image (easiest is to click on Custom and enter original height plus half original width)
click on the middle of crop region (hand grab icon) and drag the image all the way to the RIGHT
click again on the dotted rectangle
X_1 will now be an image that is just the LEFT half of your original image.
STEP 3 - In the Develop module, then take X_2 and:
click on Crop Overlay
select the same region that you used for step 2
click on the middle of crop region (hand grab icon) and drag the image all the way to the LEFT
click again on the dotted rectangle
X_2 will now be an image that is just the RIGHT half of your original image.
STEP 4 - In the Library module
select the folder your original photo was in
find your two new copies of that photo
drag them to your book situated in the Collections folder on the left menu bar
STEP 5 - Look under Collections and click on your book and then in the Book module
in the Background section (right side menu), UNcheck "Apply Background Globally"
to the right of the small rectangle below that, click on the tiny arrowhead and make sure "Photos" is selected
select the left page of your spread and drag X_1 (the left half) onto the rectangle
select the right page of your spread and drag X_1 (the right half) onto the rectangle
DONE!
Of course, you can now select any of the existing templates for your left and right pages.  Remember, if you don't fill a particular template frame with a photo, it remains blank in the finished product (you can always check the preview in the top left corner in Spread View mode).  For example you could pick a 4 x 4 template and only use the top three outer frames on the outer edge of both pages.
Have fun!  I was so excited when I finally worked out this work-around

Answer 1

I have an even simpler method. I use MyPublisher to print my books. I select their page layout for a two-page spread, drag and drop the image. DONE!  I understand that most people have their favorite places to have work done. I have been extremely pleased with MyPublisher.com, so I haven't even tried the book module in Lightroom.