Seanan McGuire's Incryptid series. Book 9, Imaginary Numbers, ended on a HECK of a cliffhanger. I reread the series when I recommended it to someone, and thought maybe the next book was due soon. When I checked what book was coming out, book 11 is available for preorder. Which meant book 10 snuck out while I wasn't paying attention! I devoured it in no time flat... and it was wonderful. I loved it so much, my husband actually started reading the series, and got completely hooked as well.