Warriors (Erin Hunter). Starless clan

"The dawn of a new age is upon the warrior Clans, but when tragedy strikes RiverClan, one young medicine cat must use her gift to prevent the Clans' newfound peace from descending into chaos"--

"The warrior code now allows a cat to formally change Clans, but ShadowClan's newest arrival feels increasingly unwelcome--and though ShadowClan warrior Sunbeam thinks every cat deserves a chance, one of the loudest voices of opposition is her own mother. Meanwhile medicine cat apprentice Frostpaw scrambles to help RiverClan stay afloat under the watchful eye of a second Tigerstar, and the shadow of growing conflict looms over them all..."--

While Frostpaw takes refuge in ShadowClan as Splashtail seizes control of her home, Nightheart returns to ThunderClan and to a complicated reunion with his mate Sunbeam whose efforts to fit in as ThunderClan's newest member are hampered by her treacherous mother.