Written and set around the late 60s/early 70s. Members of "The Agency" (FBI/CIA type organisation) are investigating the origins of spies who have capsules or something implanted in the back of their neck, and who melt when captured. Eventually they discover they are aliens, scouts for an invasion fleet. The humans also meet good aliens who help earth build their own fleet, which meets the enemy in the outer solar system in the final chapters.
This seems very similar to The Invaders, a TV series that ran in 1967 and 1968. Aliens had invaded Earth, and been discovered by one man who spent the series trying to get others to help him fight them. The aliens (almost all) had a deformed pinky finger, and in a couple episodes were seen in their alien form or reverting to that form -- an effect that might have been describe in the novelization as "melting".
What you read was probably the novelization, likely a collection of several episodes in a single cover, similar to the novelizations of other popular SF series such as Star Trek or Voyage to the Bottom of the Sea.