Question: This is not a technical question. However as the FAQs say this place is about

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> people who love to create well-structured and beautifully typeset documents
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> 

I take my shot.

I am often confused when which font family is more appropriate. Most people would suggest: "take what you like the most". But there are typographicaly reasons and I don't want to pick a font familty randomly.

I am looking for some good paper reasoning about the choice of font family, no luck so far. Are there good sources?

Answer: 
a good and detailed answer can be found also here - although it's talking about web design, I think these are good guide lines also for printed documents:
<http://webdesign.about.com/od/fonts/a/aa080204.htm>
I don't think there's a reasonable answer to this question. It all depends on typeface. In the earlier days of last century, Sans serif were mainly used for display purpose, displacing Didot like faces, and serif faces are used for continued readings. However this line is increasingly blurred thess days.

Now, you can have sans serif such as Scala Sans or Syntax, Crosnos and many other faces that are as humane as as any serif face. 
Legacy Sans is as much Venetian as any serif faces I have seen. And they are very well suited for text usage.

On the other hand, there are also sturdy serif faces that blur the lines between sans serif and serif faces. And some serif faces play the role of a display face as gracefully as any sans serif. Higher contrast faces such as Questa Grande has long played the role of a display face. If you look at any 19th century book title, you are likely to find one in Didot, Bodoni, Walbum, etc.

There's also no reason that sans-serif being more "informal" than serifs. Acumin can be as formal as any serif I know of. Jenson can be just as warm as any sans. 

The only place that sans serif is really dominant that I can think of is signage.