My rating system: Light - may contain light spanking by hand or soft paddle or simple bondage of hands or ankles. Usually the sub is new and just learning submission. Good place to start for new BDSM readers or those prefer a little out of the ordinary without getting kinky. Mild - May contain some spanking including for punishment with or without paddle, bondage such as shibari, nipple clamps, restraints including with spreader bar. Moderate: May contain bondage, flogging or caning, single tail, spanking including for punishment, other punishment acts, clamps, sharing, sadism/masochism, age play. Hardcore - Almost anything goes, probably multiple times.
*****
The Rescue Me Saga by Kallypso Masters. Hardcore. Should be read in order. The Boss series by Abigail Barnette. Hardcore. Should be read in order and features the same H/h throughout. Wanting Moore by Lauren Gallagher. Mild to moderate, polyamory. 2 books included, Light Switch and Reconstructing Meredith, feauturing the same core group. The Incorrigible Annabelle Spencer and Annabelle's Awakening by Louise Taylor. HR. Moderate. Captured Innocence by Kennedy Lane. Light. Hard Wear by Marilyn Lakewood. Light to mild. Masters of the Shadowlands by Cherise Sinclair. Moderate. Each book can stand alone although a few references may be made to previous events or people. My favorite is If/Only, an m/f/m menage, co-Doms with one sub. Polished and Swept by Alyssa Turner. Moderate to hardcore. M/m/f menage, one Dom with two subs. The Billionaire's Pet by Ivy Lane. Moderate. Little Dancer by Brianna Hale. Moderate. Dom/little. Bound to Submit by Laura Kaye. Moderate (mostly mild but does contain flogging.)