Noted. There are 3 major factors in this:

One is that Liu Xi somehow obtained the book, possibly from Xander's collection. She knew it as dangerous, so she researched a way to neutralize it - in other words, she did consider destroying it. But then she discovered that bad things happen to people who try that, so instead she looked up a "One Fifth" spell so it would be small and not so damaging or corrupting. Plus when it's very small, nobody would really read a book the size of a silver dollar very closely, so if someone sees it, that's not much of a concern.

Two is that she doesn't want too many people to know she has it, either because they'd become angry, or because they would want to take it away and put it somewhere she believes is *less* safe (or in some cases, someone might want to take it from her and abuse it). For instance Hatman or Visionary might want to stick it in a vault under the Lair Mansion when Liu Xi knows very well that things have been stolen from there before. Burying it is an equally hazardous idea.

And third, is that she really doesn't want it. She's still researching how to get rid of it, but she believes she's stuck with it until she can figure out how to make it go away permanently. She's afraid to give it to the Shoggoth because its presence might harm him, and because she doesn't really want to explain in detail what happened. If she has to explain, she'd rather be proud of having gotten rid of it already.