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.
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..