What makes a real beach read
Two things. The plot has to move fast enough to survive interruption. The prose has to be good enough to reward attention when you have it. Most marketed-as-beach-reads fail the second test. The picks here pass both.
For the airport-to-beach commute
Lee Child's Make Me. Andrew Gross's 15 Seconds. Either finishes in two beach days and keeps you from staring at your phone. Both fit in a beach bag, both are available in mass-market paperback, neither requires you to remember complex character names if you fall asleep in the middle.
For the South-of-France daydream
Elizabeth Adler's The Last Time I Saw Paris and The Hotel Riviera. Both are aspirational-expatriate fiction set in places you wish you were. Both are easy beach reads with more than enough plot mechanics to keep going.
For the comic vacation
Janet Evanovich's pre-Plum romances (Love Overboard, Thanksgiving) are the right pick for the reader who wants something genuinely funny. The Stephanie Plum series itself (Tricky Twenty-Two) is the safer pick if you have not read Evanovich.
For one literary summer book
Rita Mae Brown's The Sand Castle. 200 pages, Maryland Tidewater, three generations of a family on Chincoteague Island for one afternoon in 1952. Reads in a single sitting on a beach chair and rewards the attention it asks for.
For the rainy beach day
Kerry Greenwood's Phryne Fisher (Murder on a Midsummer Night). Rhys Bowen's Royal Spyness (Malice at the Palace). Both are cozy historical mysteries that work for the patio rather than the chair, with the kind of British wit that thrives on a fifty-degree Maine afternoon.