Like The Time Traveller's Wife, The Host uses science fiction concepts as a way of exploring relationships: here the Stockholm-like relationship between Wanderer and her re-emergent host personality, Melanie; their feelings for other humans as well as Wanderer's relationships with her own race and with humanity - a version of the double agent's dilemma. The novel centres on the love triangle that develops between Melanie, Wanderer and Melanie's former lover, and as you would hope the book has a lot of fun with the emotional chaos that results.
The Host does ask you to believe at least six impossible things before breakfast, such as the human rebels, particularly those who have loved Melanie, coming to accept Wanderer so quickly. That's a large leap - and is handled skillfully, with different characters behaving in line with their own backstories. So the colony doctor is jealous of the aliens' power to heal the bodies of their hosts; while the colony's de facto leader is motivated by curiousity. Even with acceptance, Wanderer's bizarre situation is never forgotten or put to one side.
Wanderer and Melanie are both unusually strong characters - both more determined, open minded, stoic and adaptable than others of their respective species, and this helps to justify the way their relationship develops. Their exchanges are surreal and delightful given the parasitic situation - when circumstances force Melanie to assist Wanderer she exclaims "Us girls have to stick together!"
I've been reading this book in public but I've not seen any other male readers in my travels. Souls could be seen as a feminine alien race at least in their language which is pleasantly technobabble-free: the planets they have previously conquered include "The Planet of the Flowers" and "Singing World" and they give themselves names such as "Sunlight Passing Through Ice." They even have medicines called "No Pain," "Heal" and "Inside Clean."
This book has been described elsewhere as Stephenie Meyer's first novel for adults - this is only partly true. While both the main characters are adult - Wanderer has lived many lives, Melanie a particularly eventful one on the run, Wanderer is new to human sexuality and is overwhelmed by it; this and also her quest for identity and independence from the Souls are very much adolescent themes. Of course these themes don't necessarily stop being relevant as you get older and as an adult I found this book well written and satisfying.