Are there any reasonable airlines in Spain to book travel between the north and south?

Yes! SpanAir and Air Europa share many routes & destinations with Iberia. Most flights are via Madrid (so you may have to change in Madrid). If you want to get the best prices you should book e-tickets on the Internet. That way you just show up at the ticket counter with your ID (DNI or Passport) and the Credit Card you used to buy your tix and they give you a Boarding Pass.

Iberia has direct service Sevilla to / from Barcelona (been there, done that) and also offers on-line e-ticketing at reduced rates.

Both SpanAir and Iberia offer 'Visit Spain Passes' on which you can buy flight segments (minimum: 3) reasonably priced: For SpanAir you have to fly to Spain on a 'Star Alliance' carrier; for Iberia I think you have to fly Iberia to or from Spain. We did Iberia when they required a 'closed circuit:' Madrid-Sevilla; Sevilla-Barcelona; Barcelona-Santiago & Santiago-Madrid.

Check Iberia for all days and hours in the range of your requirements.

We flew from Granada to Barcelona in early June for 25-30 Euros each. They had Madrid to Seville also.

The fare could vary greatly from day to day and hour to hour. Check them all and then make your plans.

It's just as cheap as the train. We flew from Malaga to Barcelona for $32 one way in May.

I flew Madrid-BCN for 26 euros (Spanair) and BCN-Seville for 44 euros (air europa). this was cheaper than the train. Definitely worth looking into.