How to make sense of font metrics?

Question

If I have a Tex font, and I want to really understand how it will be laid out by Tex, and what features of the font to bear in mind when tuning my output to be beautiful, what tools are useful for this?

1. I've heard that the usual measure of font size, points per em, is tricky. Is there a good overview of how these measurements are used in font design? One that relates this information to Metafont and True/Open Type fonts?
2. I'd like to see where baselines and the em square are on the glyphs of a font. Are there tools to do that?
3. Where can I get summary information about fonts? How do I understand the content of .tfm files? Are there tools that generate these summaries?
4. Is there relevant information in fontspec?

I've asked this question as a generalisation of How to determine the true size of a font, because the narrow discussion there didn't really settle all the issues.

Answer 1

Here is an attempt at answering item 2, assuming you are using a traditional TeX or pdfTeX format with TFM-based fonts (not XeLaTeX or LuaLaTeX):

For plain TeX, there is testfont.tex. For LaTeX, the fonttable package does something similar. For context, there is the \showfont command.

PS: I have no idea how to answer anything else; 1 and 3 are much too general for an answer that is shorter than a medium-sized book, and I know nothing of 4.

Answer 2

The font metric (tfm) file contains only metric information about the font. It does not contain information such as the shape of the glyphs. It contains only the information TeX needs to know where to place characters in the font (glyphs) on the page.

Here's a program that makes TFM files less inscrutable.

$ tftopl --help
Usage: tftopl [OPTION]... TFMNAME[.tfm] [PLFILE[.pl]]
  Translate the font metrics TFMNAME to human-readable property list file
  PLFILE or standard output.

I think that most of your questions involve information that is not in the TFM file.