Holy hell, this series.
Allow me to summarize my own way.
Book 1: Boring and innocent but beautiful college girl crushes on her professor. Professor lusts after dull college girl who is terrible at public speaking but is also super duper hot (all the boys want Penny) and makes all of her speeches about how much she wants him to give it to her.
They hook up. It's naughty! But hot! But naughty! And Professor Hunter (never to be called anything else, like his actual name for example, because we need to remember this is a risqué relationship) is keeping secrets but they love each other like crazy (for real, like CRAZY) but then after immediately wanting her to move in with him, he learns his assumption about her age was wrong and "needs time" to figure things out.
Book 2: Professor Hunter hasn't spoken to Penny in weeks and won't respond to any of her attempts to contact him. Her birthday passes with no word. She sends him a "goodbye forever" text and goes to a Halloween party as sexy Poison Ivy to get over the professor and ends up sleeping with her good friend Tyler (who tried to sexually assault her in the first book, but that's totally forgiven). The professor shows up, deduces that Penny slept with Tyler, and they brawl. Penny gets clocked in the head and ends up with a concussion. Professor Hunter (even though they've been banging constantly she still only calls him "Professor Hunter" instead of James) takes her to his apartment to care for her and admits that even though he never thought they'd broken up, he slept with his soon-to-be ex-wife because...well, it's not really clear why, because he apparently loathes her? Penny is understandably upset at his shameless hypocrisy and goes for a walk only to collapse and end up hospitalized. They have sex in the hospital because Professor Hunter loves her! He meets her parents, who believe the lie that he is a fellow student at the college. But uh-oh! Isabella, the scorned wife, leaks the news that Professor Hunter and Penny are sleeping together to the college and the media. Consequences ensue. James (I can't do it anymore) resigns to be with Penny. Everyone is horrible to Penny except Tyler, who is also in love with her, and James. James proposes to Penny after two months of screwing like bunnies in the coffee shop where they first met. She says yes, because she loves his sexy eyebrow hunger. Oh, and this is also the book where Penny learns that James is an addict.
Book 3: We're skipping ahead two and a half years. They've moved to NYC, and Penny is graduating. Penny and James are finally tying the knot, but his parents refuse to meet her, and when they finally host an engagement party for the couple, they invite Isabella. Penny is desperate for his parents to like her, but instead they offer her millions of dollars to stay out of their son's life for "his protection." She refuses and tells them what terrible people they are. Isabella confronts Penny and warns her to back off or else, then slaps her for good measure. James and Penny start a competition to see who can go the longest without caving and begging for sex. James is whisked away to his bachelor party in an undisclosed location, and Penny starts receiving anonymously delivered envelopes with photographs of James kissing other women (this series is aiming to cover all of the cliches). She worries that James is addicted to sex, and will get it elsewhere since they didn't do it for the week before he left. She cries and stresses the whole time he's gone, but when he gets back she doesn't mention the deliveries. James admits to Isabella kissing him but said he warned her off and now she's so helpful and friendly! (Side bar: he KNOWS Isabella smacked Penny, but apparently even though he's super protective and possessive - seriously it's mentioned multiple times per book - this doesn't mean she can't be a lovely person! He also knows Penny is insecure about Isabella but pish posh!) Isabella blackmails Penny with explicit sex photos into stealing twenty million dollars from James, but then warns her she can't tell him or else she'll release the photos anyway. James confronts Penny, and since she's freaked about Isabella knowing her every move, she doesn't tell him Isabella is the evil mastermind. James is furious because Penny only wanted him for his money, so she could run away with Tyler (who got the 20 mil), and he says a bunch of really messed up shit and tells her to leave him alone forever. Penny does leave. The wedding is off!
Book 4: The wedding is on! What if James hurts himself? Penny can't leave! Tyler pledges his love and begs Penny to run away with him, but she refuses because JAMES. Even though he's repeatedly lied to her while stressing the importance of no secrets between them, and said all of the terrible things he said, Penny immediately forgives him after some makeup sex. She told James' parents about Isabella, and she thinks his dad might have a soul, but mom is still firmly on team Psychopants even though there's evidence that Isabella stole 20 million dollars and all mommy wants is to protect James from gold-digging whores! Or possibly regular people who have the nerve to be middle class! There are jokes about Rob giving Penny roofies, because sexual assault is funny! Anyway the rehearsal dinner goes on as planned, but the parents show up, and Mommy Hunter is vile as always. Daddy Hunter takes Penny aside to confess his mistakes and welcome her to the family. More sex. More worrying about whether or not he's addicted to her, or he really loves her. The wedding happens. James is shot and nearly dies! Before they get to consummate the marriage! Penny discovers she's pregnant, but James doesn't want kids. But he's in a medically-induced coma! It's all very stressful. Meanwhile, they know Isabella tried to kill him! Which makes no sense, because apparently her motivation is to get him back? But she tries to kill him twice. And fails. James eventually wakes up, and Penny is mad because he's still keeping secrets four books later! GIRL, the man is just going to keep withholding because otherwise these books would just be sex scenes and far-fetched murder attempts. Then Isabella tries to kill Penny in the hospital room but a bodyguard bursts in at the last second to save the day. Isabella is dead, and they can finally be happy!
Penny is BORING. She has zero personality aside from her unconditional love for James. She actually thinks to herself in the book that there is no her without him, because she has no identity outside of her love for him. SEE A THERAPIST. He does! The drama is contrived. The sex scenes aren't as hot as they could be because Penny and James really don't have a connection outside of sex, and some kind of emotional connection would really help. Two books were more than enough, and that is evident because of the third and fourth. However, I would've thought four would be the absolute maximum as far as being able to squeeze any kind of plot out of this series. There are four more. FOUR. MORE. BOOKS.
How?