Please note I'm not a teacher.
Both means "I don't know where, unspecified, different to that where we are or that mentioned" but in case of elsewhere it may means in a few places at the same time.
I think your example with book isn't the best one.
You will find more information on the diet elsewhere in this book.
The first sentence might imply that the information is in a few chapters or all over the book, but you still don't give any information where.
You will find more information on the diet somewhere else in this book .
This simply states that there is more information about the diet in the book, presumably in one chapter.
But in that case, I think, you would be able to be more specific.
The example with glasses shows exactly the difference : you couldn't put your glasses, one thing, in to a few different places at the same time.
Let's go elsewhere -I think it might be a bit ambiguous.
and another sentence :and are described in detail elsewhere , e.g. Wells [ 1992 ] and Keenan [ 1992 ].
It can be found in Europe and elsewhere.