is it illegal to read or write sex stories involving underage girls?

Depends on your country. If you're in the US, no problems, you're good to go -- no videos, including animated; no art or photos, but text is fine. If at any point, however, you think you may be growing more attracted to real children as a result of the fics, well... that's... bad. Really bad. Children are on the do-not-screw list 5ever. If any hypothetical attraction increases, you should destroy your stories, purge all evidence, delete your bookmarks, keep yourself busy, seek therapy if feasible (not all therapists can be trusted!), and most importantly, avoid contact with children. (Wish they taught that in schools. You probably don't need it, but you're pretty high on DDG, and I figure someone reading this thread someday'll want it.)

Of course, if you have a lower-level fetish for the scenes described, with little to no desire to actually enact them, and strong morals, and you live in a country where it's legal -- absolutely you can. Good luck; we'll be waiting.