First of all, I think gifts are always that need to be something that comes to your mind when you think of that person, it needs to suit them as an individual. I don't know how much time you spend together or how much non-job-related conversations you have but if she ever mentioned that she liked something ESPECIALLY or REALLY wants to get something, these are the first things to look at.
However, as an artist, I always find it difficult if people gift me art supplies because I prefer to get it myself since I know exactly what I need and always feel guilty when friends just give me something that I actually have no use for.
You also can try and get a book on her favourite artists, or photo books with things that interest her.
The next thing is to gift something personal and that means... research, I guess. Look through her portfolio, look at her interests, and maybe (big fat maybe here) consider creating something (I guess writing in your case) for something she loves. Most artists would be over the moon if we got fanfics for our OCs or even our favourite characters from that one show we're obsessing over.
And yet another thing, if you're meeting up, treat her to a really nice meal/have cake/something self-made (please don't do this if you are a terrible cook, though) - also a bouquet of flowers can be really nice even if it sounds very cliché because most people just don't buy them for themselves.
And, this might sound a bit weird but since you're working on a series together, maybe create a mixtape that works with your baby, like the general mood of the series or songs that represent certain settings/characters etc. You can do the same but with songs that you might associate with her art.
Uh. Yeah. That's what I came up with right now, maybe it's somewhat helpful...?