Hmm…well, Laurell K Hamilton’s very first Anita Blake book is a great mystery novel (with no sex! promise). It’s called Guilty Pleasures, and Anita Blake is a very strong female character. (Up until The Killing Dance, there is mention of sex (and lots of violence!), but no actual sex until then.)
For older readers, Anne Bishop’s The Black Jewels Trilogy and James Hetley’s The Summer Country are both good, but, again, there is lots of sex and violence.
Tithe/Valiant/Ironside are all good books by Holly Black that are somewhat shorter than some of the others I mentioned, but are very good. Although I wouldn’t call either main character a “strong” female, both have to make difficult choices and deal with the consequences. (Tithe was written first, Valiant takes place in the same universe but with different characters, Ironside is a sequel to Tithe.)
Wicked Lovely (Melissa Marr) is good and has a female lead, but again, she’s not so much strong as having to deal with her choices.
Charlaine Harris writes some wonderful books, usually with strong female leads, including her Southern Vampire series (first book is Dead Until Dark) and some mystery series (Grave Sight, Real Murders and Shakespeare’s Landlord).
Annette Curtis Klause’s Blood and Chocolate and The Silver Kiss deal with werewolves and vampires, respectively, and have female leads.
I liked Maria V. Snyder’s Poison Study, but her subsequent books aren’t as strong.
Have you considered a biography of a strong female, such as Julia Child or Marie Curie?
(Or even Julie and Julia?)
Sharon Shinn writes good female characters, but boy is there angst! Archangel would be a good place to start.
I really liked Fly by Night, by Frances Hardinge, for a very strong female lead and interesting premise. It is, however, for younger readers, although a fellow friend in her late 30s enjoyed it immensely.
A Nameless Witch, by A. Lee Martinez, is good and interesting with a strong female lead.
(I’ll mention Kushiel’s Dart, because I am an enlightened sexual female, but it’s probably too long and involved for any class.)
(Do you want more????)