If anyone else is going to read Nona, I strongly recommend you read it immediately after Harrow and immediately before Alecto. And keep a Pynchon-style notebook. Neither I nor a friend could make heads or tails of what was going on without drawing a diagram.
Gideon is amazing. Harrow is amazing. Both are good casual reads. Nona is far more demanding of the reader.
* The Locked Tomb Trilogy -- the first book is so good and the second and third books are good and unique.
* The Great Cities Duology (first book was stronger)
* The Paladin's Legacy (sequel series to The Deed of Paksenarrion, great to be back in that world)
* The Founders Trilogy by Robert Jackson Bennett
* The Up and Under series by Deborah Baker is super whimsical
* The Redwinter Series by Ed McDonald
If you're into the genre, all of these are great (at least the ones I've read, haven't finished all the series).