Section 4.2.2. in Notes on book design makes a reference to mixed portrait and landscape pages.
I am aware of
pdflscape, that allows such a mixing, but as this manual is written by the author of
memoir I would assume that such a scenario should be achievable also using the built-in capabilities of the package itself.
I am unable to find any references in the main manual, and would appreciate any comments pointing me in the right direction, as I am sure I must have been overlooking this feature.