DocsX11R7.7 comes with extensive documentation in the form of manual pages and typeset documents. Before installing fonts, you really should read the fontconfig(3) and mkfontdir(1) manual pages; other manual pages of interest include X(7), Xserver(1), xset(1), Xft(3), xlsfonts(1) and showfont(1). In addition, you may want to read the X Logical Font Description document by Jim Flowers.
The comp.fonts FAQ, which is unfortunately no longer being maintained, contains a wealth of information about digital fonts.
Xft and Fontconfig are described on the Fontconfig site.
The xfsft home page has been superseded by this document, and is now obsolete; you may however still find some of the information that it contains useful. Joerg Pommnitz' xfsft page is the canonical source for the “ttmkfdir” utility, which is the ancestor of mkfontscale.
The author's software pages might or might not contain related scribbles and development versions of software.
The documentation of X-TrueType is available from the After X-TT Project page.
While the Unicode consortium site may be of interest, you are more likely to find what you need in Markus Kuhn's UTF-8 and Unicode FAQ.
The IETF RFC documents, available from a number of sites throughout the world, often provide interesting information about character set issues; see for example RFC 373.