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>
There is one reason to use serif and not sans serif:
It is less likely to read a "l" as a "I" or vice versa.
In the other hand, the "1" and the "l" look very similar on some serif fonts, but still better than the "l" and the "I" in most sans serif fonts.