A good guide book such as the Bradt Travel Guide is also very useful.
Visiting Dubrovnik from Hvar may be a bit difficult - there are ferries departing Stari Grad, but journey time is 7 hours, so unless you want to stay in Dubrovnik, you wouldn't get much time there (and ferry times may not be suitable).
If you get the early bus to Sucuraj you can take the ferry to Drvenik on the mainland. From there the buses run to Dubrovnik at least once an hour during the summer, taking about 2.5 hours. Or you may well be able to hire a car and drive via Drvenik which would cut the journey time down to manageable proportions.
Try the local Atlas office near the harbour in Jelsa and near the harbour at Hvar. They are helpful and organise trips to Dubrovnik. Went from Hvar to Dubrovnik with them last Tuesday and to Sibenik and Krka with them on Thursday. They are really good. Have a great time.