This is a great typographic reference.

Free e-texts on typography

This is a collection of links to some of the significant free electronic texts on typography which are freely available on-line which I have found or been told of. There are additional pages for certain topics, e.g., texts on TeX---a few of general interest are listed below and there is an additional page of free texts for that system. Other topics with their own pages include technical reference, typeface design and style guides.
[more]

  thanks to Coudal Partners

This is from one of the free e-texts. The first half has great information on book design:
Memoir is a new documentclass for LaTeX intended for typesetting books. The first half of this wonderful text covers basic design and page layout concepts every graphic designer and typographer should know.

The Memoir Class for Configurable Typesetting User Guide [It's a PDF file.]

Widows and orphans

Inconvenient page breaks can also cause a hiatus in the reader's perusal of a work. These happen when a page break occurs near the start or end of a paragraph.
     A widow is where the last line of a paragraph is the first line on the page. The term is sometimes also used to refer to when the last word in a paragraph is on a line by itself. A widow looks forlorn. As Robert Bringhurst said, 'A widow has a past but no future'. Typographically, widows should be avoided. Especially to be avoided are widows that are the only line on a page, for example at the end of a chapter. Five lines on the last page of a chapter is a reasonable minimum.
     An orphan is not nearly so troubling to typographers as a widow. An orphan is where the first, one or two lines of a paragraph are at the bottom of a page. Bringhurt's memory trick for orphans is, 'An orphan has a future but no past'.
[more]