Johnbucket is totally right. Scholars in modern Sonata theory are not the type of people populating a site like this. However, I am also not that convinced it would be useful for “Young Composers”. Sonata theory tries to analyze music and distill certain general properties for certain types of music. They do not try to follow the way a piece was composed or what the composer was thinking when he chose certain paths for the music in a sonata.
There are scholars that try to do that, and some of them lived in the hey-day of classical sonata. One of them was Heinrich Christoph Koch, a German musician and author, who compiled a very popular music lexicon and also a multi-part treatise on composition. What he writes is corroborated by other musicians and writers of his time. Now, what he wrote wasn’t on the cutting edge of musical change, but as he published his treatise in the years 1782-1793, is very useful for understanding the compositional strategies of many composers in the second half of the 18th century.
Some friendly soul at the publisher Cengage has chosen an excerpt from Koch as sample pages for a publication, so here is a small part translated into English: https://www.cengage.com/music/book_content/049557273X_wrightSimms/assets/ITOW/7273X_44_ ITOW_Koch.pdf