Actually, on page 6 there is a bit of clarification. It says, "After character creation, no single ability die can be increased more than one rank per level of advancement". You can do multiple increases in the same ability, just not twice to the same ability die tier.
Hmmm...the way it's written does leave it open to interpretation. So, if one were to rule that you can only pick one rank within an ability branch or tree, the rules would seem to support that. Likewise, if you were to do things as I do, the rules support such by omission of contrary rules.
I think I'll include this in the FAQ.
'No single ability die' seems to say that you can increase each die in an ADC once per level...
However, it sounds like you're saying that an ADC can only be increased by one rank per level... I.e. if you have:
You could increase the long sword mastery and gain a new mastery (since they follow different ADCs) OR
You could increase the swords specialization and learn a new specialization (but not increase or take a new mastery based on swords) OR
You could increase your melee and that's it for the ability (no specializations or masteries, since they all use melee).
I think it depends on how quickly you want to allow progression... I personally would suggest the one die per ADC per level, since there are still ways to game the system to boost your defense pools, but it prevents those spontaneous eruptions of knowledge by increasing all (or some) of the dice in your ADC... After all, there's a big difference between 3D4 and 3D6 when it comes to rolling harm...
However, if you want characters to be able to quickly fill in those gaps that they see from character creation or potentially change character concepts mid-way in the game (multi-class), use the increase each die once per level route...