Yep, the first offical comic collection Over The Garden Wall: Tome Of The Unknown containts the backstory of the Highwayman and his talking horse Fred!
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https://www.amazon.com/Over-Garden-Wall-Pat-McHale/dp/1608868362
My two year old also LOVES Hilda and her favorite book is the one with all the monsters and spirits in it. I’d say go ahead and invest in the books. :)
Edit: Here’s the book I mean: Hilda's Book of Beasts and Spirits (Hilda Tie-In) https://www.amazon.com/dp/1912497565/ref=cm_sw_r_cp_api_glt_fabc_5E6ZQME1ATRH13PXEAT1
Wonderful! Thank you so much, and please correct me if I'm understanding this incorrectly.
Originally there was a special issue and a four-issue miniseries, which has since been collected in an anthology of sorts, called Over the Garden Wall: Tome of the Unknown.
Similarly, the ongoing series is shipped individually each month, but there are "volumes" available which contain multiple of these comics bound together? Those volumes are the two amazon links you've provided, with the yet unreleased volume 3 being at this link.
And then there's the second special, the naming of which confuses me because it's called "2017 Special #1", which may or may not end up being included in volume 3.
Am I incorrect in any of this?
Something I feel like I should make note of first: The graphic novels and the show have two different geographies. If you look at the maps from the graphic novels planning on comparing them against the ones from the series and series tie-in books, you will be sorely disappointed.
With that being said, I would highly recommend getting (Hilda's Book of Beasts and Spirits)[https://www.amazon.com/Hildas-Book-Beasts-Spirits-Hilda/dp/1912497565/ref=sr_1_1?crid=3RRHULYUNTTCU&dchild=1&keywords=hilda%27s+book+of+beasts+and+spirits&qid=1629868054&sprefix=hilda%27s+boo%2Caps%2C198&sr=8-1]. It has the largest contiguous map of any that I've seen in the series or the graphic novels (I have not read the tie-in novels, so I can't vouch for them), it is based of the geography of the series, which means you can far more easily compare all of the series maps against it, and having reference information for most of the canonical creatures is quite valuable for a campaign. If you want a more detailed map of Trolberg, the best one I know of is in S1E6 at roughly 4:50. There's also a map in S1E10 at 5:49 which I have speculated is a map of the continent with Trolberg located at the north east corner (I have no other proof this is true than the fact that the bay in the north east corner of that map looks a bit like the Bjorg Fjord).
You didn't ask, but I personally think Tales From the Loop would be a fantastic ruleset to build a campaign off of. It's pretty simple to learn and run, and its designed so that all the Player Characters are 10-15 year old kids.
Doing a bit of research I found the book on Australian Amazon for $22.75 + shipping. Unfortunately it's extensively more expensive to purchase things from online retailers that have to ship all across the globe.
As far as I understood it, this was a one pager that Katie O'Neill did on her website but then went on to turn it into a full series and had it printed.
A few of the redditors above mentioning the hardback book found on Amazon, a great way to support the artist.
All of the comics are canon, but I think I know what you mean. The first 5 take place directly during the show (4 about the main gang, and 1 about the woodsman's daughter), so they are the most canon I suppose. Those 5 are now in an actual book, which I really recommend, it's very nicely put together. The on-going series (Greg's dreams, Wirt's dreams, the Woodsman's daughter, and occasionally Langtree's school) can be found at about half of most comic book stores, I've been to around 7 or so shops lately, and had luck at 3 of them.