Hello Irene here again!
I wondered if any body knew if there is a bus direct from Iguazu Falls to Salta? How long the journey would be (I can imagine long) and the cost? I will be travelling in April. Also how far in advance do you have to book buses (especially over night and weekend) can you generally book them the same day or at least a day in advance.
No direct connections as far as I know, but there's a number of interesting ways to do this, speaking from the top of my head.
A nice one is Iguazu - Ciudad del Este (PAR) - Asuncion - Formosa - Salta; it'll take some time and some changing busses, though. On the other hand, Formosa province in Northern Argentina is very interesting.
A more feasible alternative involves the southern parallel route: Iguazu - Posadas - Resistencia - Salta. Less feasible (and quite a detour) would be routes through Paraguayan Chaco and then approach Salta from the north, as well as travelling all the way south to Santa Fe and then up north again to Tucuman and Salta.
None of these routes is particularly busy, though, so there should be no need to book more than a day in advance. I've slightly more worries about connections; allow for a few days of flexibility if you're planning this kind of itinerary.
have fun and good luck,
I did the opposite way. From Salta to Corrientes (15 hs) bus company "la flecha" (good one), the another bus from Corriente to Puerto Iguazu, bus name...i don't remember it but it was the worst.