Proceed With Caution:
This book contains mentions of dog fights, drug use, child abuse, slavery, and violence and gore.
Willa's Beast takes place simultaneously with the previous two books, but can stand alone. I'd still recommend reading the other two because they're fun. This is Willa and Gren's book, who we already know take off from the beach tribe, yet no one knows why or where they went.
I was extremely curious about Willa and Gren after reading Veronica's Dragon, so I'm glad their book was next. Although, I do feel like it would have made more sense to have been book two than book three. Willa's Beast does carry the timeline forward further, but I feel like the previous book "spoiled" this one a bit. Nothing major, but I do think this story would have been a better fit earlier in the series.
Willa's Beast was quite different from the other books, at least to me. Normally, it's been the male stealing away his mate to complete resonance. In this case, Willa steals Gren away! And they're not even resonating (yet)! She hated the way he was treated like a wild animal, so she frees him and they flee into the mountains. She's really the one taking the lead in this situation and Gren is just happy to be with her. She never wants him to feel enslaved again. Willa has her own issues surrounding being dependent on others which she has to come to terms with as well.
Willa's Beast also contains some new happenings that we haven't seen from either series yet. I was very pleasantly surprised at how the author can come up with all of these new situations that feel believable within the world to keep the series feeling fresh. This one deals more heavily with resonance that the previous books. It's always a driving force of the plot, but never the true focus. The couples have marathon sex for a day or two and then everyone is cool. Willa and Gren are having some complications though.
I really enjoyed Willa's Beast. This series keeps getting better. You'd think that after reading thirty books with the same subject matter, things would start feeling stale and repetitive. That is not the case at all, even with these first three books all happening at the same time!