Nicole Wilson
Basic
Janet Reid, aka Query Shark, a very well-known and highly respected literary agent in the US, posted this blog post about etiquette in querying your second (or other subsequent) books while book #1 is still out there. Though the aim is toward what to do if am agent has a full MS of #1, the advice in here applies to even if no one has book #1 and you want to query the next one. Good stuff.
http://jetreidliterary.blogspot.com/2015/12/querying-new-novel-while-old-novel.html?m=1
http://jetreidliterary.blogspot.com/2015/12/querying-new-novel-while-old-novel.html?m=1