Gnome Claire *Wishes she was as cool as Gnome Ann* I'd read 1&2 but not book 3 before this one, I think you definitely need to read 1 (it was my favourite of the series anyway). I felt like I was missing pretty big chunks of the story (Demi the MC from book 3 plays quite a big part) so you probably should read book 3. Casper and Anha don't play much of part in this so you probably don't need to read book 2.
Victoria I would say you can probably read this after Wicked as they Come, since the previous two full length novels are about different couples. This story follows Tish and Criminy Stain once more. You would see the other characters from previous novels too and you wouldn't know their backstory but you can still enjoy this one. :) I've read all four of the full length novels and have skipped the novellas, which I think do help but are not absolutely necessary to understanding of the plot.