I picked my list in character. For one, I figured that Portia would turn her nose up at any text that's lesser quality than she herself could produce-- she doesn't think of herself as a writer, so anything worse than what she'd write is subpar. She would fight tooth and nail to oppose any purchases of texts below Q10. For the lab texts and the casting tablet, I tried to choose utilitarian spells/effects that would be useful for starting a covenant, rather than what Portia would personally prefer, but the idea there was that whatever purchases she's making are on behalf of the covenant rather than herself.