I was NUTS about this series in elementary school. The conceit is that it's a group of kid detectives - there are four of them, with the initials T, A, C, & K. They choose T*A*C*K as their acronym/code name because weaving back & forth, or tacking (heading to starboard, then to port) is how you head in a straight line in sailing. Probably the introduction to my lifelong love of cozy mysteries, definitely my introduction to the illustration work of Alan Tiegreen, and a staple of my mid-80s monthly Troll/Scholastic book club order.