The Effects series is a collection of novels that can each be read as a standalone. Readers can easily come into the series at any point and enjoy each book as a complete story, unto itself.
Having said that, the books are linked so there will be mild spoilers if you read them in random order.
Ripple Effects and Sound Effects are interchangeable as a starting point. Ripple Effects was the first book written in the series, telling the story of Danny and Sarah. It introduces many of the characters in the Effects series. Sound Effects then takes an eleven-year step back in time to tell Jamie and Mel’s story, which is referenced in Ripple Effects. Chronologically, Sound Effects is the first book in the series, and also features a younger version of Danny (still in his dreaded cologne phase!).
Either way, you can’t go wrong if you begin with one of these two.
The third book, Aftereffects, is Keir and Selene’s story, and chronologically takes place a couple of years after Ripple Effects. Selene is a bit older now, and trying to make a name for herself as a graphic artist, when she rear-ends Keir on the Golden Gate Bridge. Danny, Sarah, Jamie, and Mel all make appearances.
And finally, Side Effects is Marcus’s story. We first meet him in Ripple Effects as a 21-year-old prickly boy-genius. In Side Effects, he’s four years older, now the founder of an indie gaming studio (thus, the slightly different cover art!), and no less prickly for the passage of time.
Side Effects contains an epilogue for the whole series, bringing all of these characters and stories together for one final glimpse into lives well lived.