"at will" is not possible in LR Book module.
There are many (~180) page templates that you can choose from.
The photo 'Cells' on the page templates can be resized, but only smaller, by changing cell 'Padding'.
For "at will" page design (eg. collage, fancy borders, etc) you will need to create page images in Photoshop or the LR Print module, and use as "full page" images in the book.
Thanks, wobertc. That's what I thought and feared. Aperture had its drawbacks, but it was very good for producing attractive books. Apple won't support it anymore and Photos and LR are crippled in comparison.
Adobe promote InDesign, Photoshop, Illustrator, as graphic arts software -I don't believe they will make the Book module in Lightroom into a fully featured design program when the others exist.
Also there are many other book creation programs and on-line book design sites (eg. AsukaBook) of varying quality and features. The Best Page Layout Software of 2016 | Top Ten Reviews
I am happy to accept the limitations of the LR book, but frequently design pages in Photoshop and the LR Print module (mainly the Custom Package option)- export these as JPGs to use in a book.
So if you have the CC package then you can make good use of Photoshop for un-restricted page design.
I have always been pleased with the quality of the books I have sent to Blurb.