This may not be a big mystery.  In Grave Peril when Harry meets Mavra at Bianca's masquerade ball, she tells Harry, "We should talk, I suspect we have much in common."  Harry says something like , "I doubt that."  Then he picks up on her aura and recognizes her as the being behind the nightmare.

So its likely that Mavra was just referring to the sorcerer Kravos; they both had him in common.  Its also possible that Mavra meant something else entirely.  Also, in Dead Beat Mavra was rather talkative with Harry; in almost a friendly way for soulless creature of darkness, until Harry told her to get to the point. 

I get the feeling there are some things about Mavra we haven't found out yet.  Others have suggested she (or it) is a BC member and that may be true.  I won't be surprised if we find out she had some connection to Margaret LeFey or Justin. 

When he animated the T-rex in dead beat, he told Ramirez that is just hard work, not complicated. Again we can argue that it is just Harry being modest, but no one other then Ramirez compliment it, and Ramirez is just a young wizard with limited amount of magical muscle. He do not have enough expirience to rate that feet and due to his own limited amount of strength, he might think that Harry's feet in animating the T-rex is impressive because it is beyond his own strength.

I seem to recall Luccio was impressed as well, if also slightly disgusted by the use of necromancy.

Some random thoughts on Ghost Story and what comes next; some of which may contradict each other.

As at least one other reviewer has said, Ghost Story is a bridge story between the younger Harry Dresden and the mature Harry Dresden.  There is no giant cliff hanger like at the end of Changes but there are an awful lot of smaller but important issues which have yet to be resolved.

For example, Mab now realizes she can’t just push Harry around like a meat puppet, but she’s Mab and she’s not going to back away from trying to mold Harry into a tool she can control.  Instead of directly controlling him or messing with his memory I think she will give him damned if you do and damned if you don’t decisions to make. 

Maeve doesn’t yet know she can’t push Harry around so their interactions should be a lot of fun.

Will Harry be able to get word out to the White Council and his friends that he is alive again?  Maybe not right away, perhaps Mab will want to use the new Winter Knight as a secret weapon on her enemies.  Perhaps HD will have to carry out a mission or two before he can get word out and go home for a visit.

What happens if Harry spends a few days in Fairy and the Nevernever?  Does he go back to Chicago and find out a year or more has passed?  At the end of GS it really felt like we were saying goodbye to a number of characters, maybe for a long time.

Who has Harry’s amulet and why, The Leanansidhe, Mab or a third party?

What does it mean to be the custodian of Demonreach?

What happened in the Bob vs. Evil Bob throwdown?

How will Felicia try to get back at Murphy?   What is her exact relationship to Lara and Thomas?  Whatever it is, she’s not going take the beat down Murphy gave her without retaliating through some cat’s paw.

Daniel Carpenter has some anger and self control issues.  Daniel’s been pissed off since Molly left home, leaving him as the oldest child of the Carpenter’s, with all the responsibilities that implied.  It appears to me Daniel wants to be one of the wielders of the two remaining Swords of the Cross.  Will someone like Felicia target him for manipulation?

Does Fitz have a larger role to play?  As someone who could hear Harry’s ghost does that make him an ectomancer?  Will he grow into someone who can wield one of the Swords?  How will Murphy react to him when they meet?

Did Abby survive the bullet wound she received? Will Helen Beckett become the temporary or full time head of the local paranet?

Is Molly really sane now?  Will the WC get off her case?

Will Karrin Murphy get any crazier or does accepting Harry’s demise allow her to regain her equilibrium, at least until she finds out Harry is alive?

Who or what is Marcone’s new troubleshooter, Mr. Childs?  Is he like Kincaid, only half human?  Why has Marcone left town in the first place?

I can come up with more questions and I haven’t mentioned the larger issues of The Black Council, the second traitor in the WC and the most important of all questions; when will Cold Days be released?