I believe that Mavra does have the Word of Kemmler, Harry gave it to her at the end of Dead Beat because she was going to black mail Murphy. He also warned her not to try to get to him though his mortal friends again, that he also had read the book and since he did raise Sue, understood it, and knew how to use it.
Note however that he relied upon Lasciel's Shadow (photographic memory) to read the book for him.
We don't know how much of that info he retains. He almost-certainly no longer remembers Ghoul (iirc that's Sumerian) as he failed to recognize/understand it in the Deeps after Lash takes the psychic assault for him. By inference, he no longer remembers Etruscan (Whamp formal tongue).
I suspect he can reanimate animals as zombies -- he did that magic
himself (even if the Shadow tutored him); dunno about human zombies, it may be different-enough that he can't (not to mention that'd violate the Laws).
But all the other stuff he got from the Shadow? I don't recall that we've seen
any of it since the Deeps (only the music he played at the end of DB, and that just the once). In particular, we don't know if he still knows any of the rest of the
Word of Kemmler. Per the text, Harry and the Shadow discussed it for "an hour or more," but that wouldn't be enough to convey the entirety of the book (I presume Harry didn't revisit the BigBadBook -- as taught by the Shadow of a Fallen Angel -- after that night).
Details of the Darkhallow, the generalities of how to use necromancy against BCV's, how to raise Sue, and the pain-relief practice. That's the specifics that he himself consciously worked with in his own mind, and presumably therefore still knows.
We know from the not-speaking-Ghoul scene that SOME of the info is gone; we know from Harry using the pain-relief that stuff he actually practiced (as his own act of will) he still has.
I think for the rest of it, it will depend on what Jim wants or needs later in the series. I suspect there will be some issues with Bonea, what she knows, how much of her mom is in her, etc.
And -- as I have WAG'ed before -- I think it's entirely possible (bordering on probable) that although "
Lash" is gone,
Lasciel's Shadow still lurks within Harry.