This may not be an exact fit (although OP admits to having a rather hazy memory of the IP in question). But I think this might be "With Interest" by David Betts - a form of puzzle-book-meets-ARG, published in (I think) 1983.
In the book's story, aliens with six fingers on one hand, and four on the other, visit Earth to find out about us, and to try to find out the meanings of the strange "pound" and "dollar" signs we seem to use so much. The story is a comedic affair, suitable for young children, with some surreal aspects, and it's possible that the book could be enjoyed at this level simply as a story for young children.
However, the story's real purpose was to contain clues for an ARG, in conjunction with ten watercolour illustrations of places in the United Kingdom, and various other illustrations and snippets of text. This would certainly fit in with OP's remembering a book, but being unsure as to whether it was really a game, or part of one. The book itself even contains a simple cipher stating "The story doesn't make sense, but it's full of clues."
To win the ARG, the ten locations had to be identified, and the player somehow had to work out that they had to go to a specific telephone box at a specific time on one of a handful of dates, and wait for a call. These dates/times corresponded to the times at which the aliens were able to communicate with their home planet.
To add to the confusion, buying a new dustjacket for the book after the ARG ended (I think 1986) would turn it into a follow-up ARG, now entitled "With Interest Revisited: The Quest For The Stone". The new clues on the dustjacket, in combination with clues hidden in the book but unused/unusable in the first game, would supposedly lead to a hidden stone and a prize with significant monetary value.
(This prize has not been claimed. Quest for the Stone was never solved. There is discussion on this and other topics related to the game at www.quest4treasure.co.uk/phpbb3/viewtopic.php?f=39)
In addition, With Interest spawned a small franchise of related media. A calendar containing new clues, and new illustrations of the ten locations, was released for purchase at some point. On three specific dates, adverts with coded messages might be placed in the classified ads sections of certain newspapers; solving these and writing to the addresses given would lead to extra clues being posted to you. Another "additional clue sheet" could be sent away for after you discovered its existence by solving a particular puzzle. Newsletters were sent out to players, and I believe one of these contained promotion for a non-affiliated piece of ZX Spectrum software called "Treasure Hunt Toolkit".
The amount of extra material published fits in with the OP remembering this as part of a larger IP.
Now, there are some ways in which this doesn't quite match OP's description:
It's never stated where the aliens in With Interest come from. Alpha Centauri is never even mentioned.
They do have six fingers on one hand - but only four on the other.
Although I think they did look human other than that, I don't remember for sure. The new dustjacket had silhouettes and shadows of the aliens seen from a distance, in which they appeared to have antennae, but those might have been appendages on their helmets.
There is a picture of the book's front cover at:
Low-res scans of a fold-out illustration from the new dustjacket can be found at:
(Despite what that page states, the With Interest sequel, Imperial Twelve, has been awaiting release for a few decades now and is unlikely ever to see the light of day.)