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How, typically, do "book" versions of typefaces differ from the regular versions? 

One would assume from the name that they are intended for the main typeface in books, just as "schoolbook" versions are intended for children's books. 

But how do these differences manifest themselves specifically? 

Berthold Baskerville Book, for example -- by the way, thanks for the recommendations, folks; it is BEAUTIFUL -- seems to differ considerably in the glyph designs from plain Berthold Baskerville. The book version seems to have more flair and panache in certain characters, while the plain version is more staid and homogeneous. The book version also looks just a touch condensed. 

Gill Sans Book, on the other hand, looks more like an attractive weight midway between the Gill Sans Light and Regular, but otherwise not considerably different in the characters or width. 

Grateful for any illumination -- thanks. 

Victor