Flatiron Five Fitness and Flatiron Five Tattoo are both set in the same fictional world. You can read the two series separately.

If you want to read in the order of events, regardless of series, this would be your reading list:

Just One Fake Date (Tyler and Shannyn)

Just One More Time (Kyle and Lauren)

Just One Night Together (Damon and Haley)

Just One Snowbound Night (Spencer and Olivia)

Just One Hometown Hero (Cassie and Reid)

Just One Vacation Night (Reyna and Kade)

Just One Unforgettable Night (Lexi and Gabriel)

Two Weddings and a Baby (a novella)

Just One Second Chance (Theo and Lyssa)

Just One Christmas Night (Chynna and Trevor)

Just One Silver Fox (Jax and Pierce)