The Xindi remain unexplored in the novels outside of Last Full Measure
, though they've made small cameo appearances, most notably in the short story "Meet With Triumph And Disaster" in the anthology The Sky's The Limit.
A Xindi-Reptilian serves as the Federation Secretary of Exploration or some such, and it's implied that New Xindus has become part of the Federation. Hopefully, later novels (or novel series?) will eventually tell the story of what happened with the Xindi between Enterprise
and The Next Generation.
The general consensus, though - in the novel 'verse and in other continuities like Federation: The First 150 Years
and Star Trek Online
, is that the Xindi have joined the Federation by the 24th Century.