G. Kroonen, Etymological Dictionary of Proto-Germanic (Leiden, 2013) outlines the phonetic changes from Proto-Indo-European to Proto-Germanic in its introduction. However, it does not outline the changes from Proto-Germanic to the descendant forms it lists under each definition:
*hardu- adj. 'hard, severe' – Go. hardus adj. 'id.', ON harðr adj. 'id.', Far. harður adj. 'id.', Elfd. ård adj. 'id.', OE heard, hard adj. 'id.', E hard, OFri. herd adj. 'id.', OS hard adj. 'id.', Du. hard adj. 'id.', OHG hart, herti adj. 'id.', G hart adj. 'id.'
I would like to figure out the phonological shifts from Proto-Germanic *hardu- all the way to, for example, German hart. What do professional linguists do in this situation?
So far, I have worked step-wise from Wikipedia – from Proto-Germanic to West Germanic, to Old High German, to Middle German to German. However, this process does not seem rigorous.
What reference works do linguists have for these changes? G. Kronnen (2013) xli contains a nice diagram from all the changes from Proto-Indo-European to Proto-Germanic:
I would like a book with summaries like this for various langauge pairs: Proto-Indo-European to Proto-Germanic, Proto-Germanic to Proto-West-Germanic, Proto-West-Germanic to Old High German, etc. etc. etc..