I would say:
- Anything by Una McCormack counts
- A Stitch In Time, definitely
- Serpents Among The Ruins or the Crucible Trilogy from DRG3
- The entire Vanguard series, start to finish - 8 books, starts with Harbinger, by David Mack
- The Destiny Trilogy, by David Mack (now in omnibus!)
Those are probably the best. A lot of the stuff that takes place after Destiny is also really good, but the interconnected continuity can be a bit much for new readers. Destiny is really accessible, though, and totally great.