Most histories of the Crusades cover the Crusader States in pretty significant detail, so they're a good starting point. I like Thomas Asbridge's The Crusades as a good starting point.

If you want a dedicated history, Malcolm Barber's The Crusader States is probably your best bet. As a disclaimer I haven't read it yet but it's near the top of my 'to buy' list. Barber is a well regarded historian best known for his work on the Knight's Templar and this book certainly looks comprehensive.

Christopher Tyerman's God's War is probably the best recent one volume work on the Crusading era. It has the marked benefit of spending a lot more time than most on the internal workings of the Crusader States, rather than the campaigns that birthed them.