First off, I found the reference on YS105 (gotta love having that PDF copy!)
Here's how I interpret the rules you referenced:
Access: You need to have a reason to be aware of the aspect in question in order to make use of it. I think of this as metagaming mechanic. For example, Donald Morgan (as statted in Our World, OW197) has the aspect Children Must Be Protected'. If your character was aware of this, you could certainly invoke it for a bonus in a social challenge to convince him that it's more important for him to rescue that child being mugged over yonder than to harrass you about questionable ritual practice (though he might invoke "Zealotry in the Cause of Justice Is No Vice" to counter). If your character had no reason to know about this aspect, you couldn't invoke it for a bonus, no matter how many children were being mugged.
Interaction: You need to come up with a good explanation for how that aspect (which is on another player's sheet or on the scene) applies to *your* action. Morgan may be a "Combat Veteran", and it would certainly be appropriate for him to invoke that aspect when fighting against a ghoul, but far less appropriate for *you* to invoke it to benefit *your* attack against a ghoul, regardless of whether or not Morgan happens to be in the scene. Basically they are just saying that while you always can make use of appriopriate aspects on your character sheet, you need to work harder to make 'foreign' aspects benefit you.
I'll admit that the third example is a bit harder to grasp, but I think that its largely a function of the aspect having very vague and/or broad wording. An aspect of "Candle-Lit" is fairly specific and would cover the first two examples, but the addition of "Ill Met" sort of opens the door to using it to justify making Bad Things happen. What can it be used for? Well, it seems to me to be more useful as an excuse for plot devices than anything else, but if the group likes an idea you come up with, then you can invoke the aspect to make it happen. (Sorry I can't be more helpful here.)
I couldn't find the "Crushed Ribcage" reference in the book, but I suspect that the example was describing a Compel, not an Invoke, and therefore plays by different rules. Alternately, there's a "Bruised Ribs" example that describes takin an advantage in melee of your target's consequence, but that example plays well with the rules for invoking aspects of others: you knew about the aspect because you inflicted it (access) and you are punching him in those same ribs to make it hurt more (interaction).