I notice from the streams that you have to choose a sub-set of your class's available skills (including ranks) to place on your single hot-bar. This raises a couple of questions:
1) Can you use abilities directly from the ability "book" without placing them in the hot-bar?
2) What are the conditions for changing the abilities on the hot-bar (in/out of combat, certain zones, requires reagents, etc)?
In regards to Question #1: I could pretty confidently guess that you won't be able to use abilities from your book - you couldn't in EQ or VG, and with discussion on limited spell/skill sets throughout development here, it would completely contradict all statements from the Dev team thus far. I'd guess there may be some clickable type items that would be used from the inventory one day though similar to Jboots in EQ.
They're looking for a limited action set, so you choose which abilities you want to utilize before starting an encounter. I would imagine that you are locked to that ability set while "in combat", and once combat falls off you're free to switch abilities in and out again. You won't be able to use abilities that are not saved to your limited action hot bar.
I agree with everyone so far that you can only use abilities that are on your spell bar. I would like to see you be able to mem abilities while in combat though and think this should require more time, maybe 2x as long. In EQ, you could swap spells while in combat.
From the latest community stream: https://youtu.be/zfxnPXjUFbo?t=702 (this is Saicred's viewpoint)
As you can see at that time in the video they needed to res somebody, but they had an enemy mez'd and they were unable to add the Rez spell into his ability hotbar and use it because he was in combat. They had to finish off the enemy first before they could put the spell on the ability bar to use it.
VR may decide to change it and have some way to change abilities during combat (using various ways to make it not abusable), but as it is currently it's not possible.