I first saw this question posted at a Tintin community a couple weeks ago, so here's my take on the matter. To be clear, these represent my best 'hangover-cures' for Tintin readers craving more, roughly-ranked from most immediately-relevant, all the way down...
NOTE: in terms of the obvious title-drops, please just google them. More specific stuff I'll link you to directly. Also note: Georges Remi and "Hergé" are the same person. Alright, here we go!
- Alph-Art, the final, half-finished album, if you haven't already.
- Lake of Sharks if you absolutely must, but it's not by Hergé.
- Ampton's heavenly review series of the albums, breaking them down and providing loads of context and insights.
- The various fan-made Tintin pastiches, if you don't mind that they can get pretty wild and ridiculous (some are French-only). EDIT: link updated.
. 
Many (or most?) of them even got collected in to two
books, by NZ art historian John Stringer, seen above.
- Then there are the earlier versions of the stories from the original publishers, Le Petit Vingtieme, Le Soir, and finally Le Journal de Tintin, i.e. "Tintin Magazine." So to be clear-- after the full stories ran their course in such works, they were later collected in to album format by book-publishers, and still later, republished with significantly improved art and some editing, here and there. What's especially cool about the early stuff is that one can plainly see the artistry and storytelling of Tintin improving bit-by-bit, not to mention see some scenes which *didn't* make it in to the albums. [N.B. you can find many of these magazines and earlier-format albums at places like Archive.Org, or the same kinds of sites where one can D/L college texts]

Le Petit Vingtieme, a newspaper extra
in which Tintin ran from 1929–1940.

Le Soir, published in the eye of the hurricane, i.e. during the occupation, 1940-1944. (eh, I'm just going to go with a pleasant "Sophia" look for this later edition, as this period wound up taking years off Hergé's life and well-being)

Tintin magazine, in which the character appeared from 1946-1976, with the journal finally ceasing production in 1993, ten years after Hergé's death. (finally, G-Remi had complete control over his own character's publication, and in terms of these covers, there's plenty of silly stuff like the above, plus many other series depicted that appeared in the mag)
- Of course, check out ANY of the many fine books digging in to the backstory and making of the series, as well as the Georges Remi biographies. For example, one can get a fascinating sense of how hard Hergé worked behind the scenes to consistently publish a comic with high credibility and integrity, eventually and directly leading to a nervous breakdown or two, along the way. 😔
- Hergé's Totor, an early template, and direct predecessor to the Tintin character.
- Jo, Zette & Jocko, a Tintin-like, 5-tome series, designed explicitly for children, altho personally I liked it a lot, speaking as a 'supposed-adult.'
- Quick and Flupke, Hergé's fascinating attempt to compete with popular newspaper/magazine strips of the day, altho I'm not sure it stands up to the test of time, very well.
- Garen Ewing's wonderful, superb Rainbow Orchid series.
- Les McClaine's superb Jonny Crossbones (FREE TO READ online).
- Yves Chaland's Freddy Lombard adventures are interesting, since they're in a similar visual format and feature a Tintin-like character, but the style is a lot more arty and mysterious, not to mention "Freddy" and friends are frequently 'punk-ass-bitches,' to express it less than artfully.
- Moving into other media, there's a program of BBC radio adaptations, two TV series, a number of movies & documentaries, seven video games, and more. [LINK]
- Oh, and of course the fake Tintin covers are huge fun! Here's an archive of 1000+ of them.
Oof, I was trying to go to Youtube to pull out one of my favorite phrases of all time such as to wrap things up, i.e. "Well, there it is!" (from Emperor Joseph in Amadeus, 1984)
Somehow, in search of that, I bumbled upon a different, beloved scene, and... oh dear, haha, I'm just gonna leave it: https://www.youtube.com/watch?v=cKPDfB40t48
UPDATE:
In the interest of fact-checking, I decided to consult with "JM," someone who I (and arguably many) consider a particularly fine Remi and Tintin scholar. Very graciously on JM's part, they provided some fascinating, thoughtful feedback that stacked upon what I'd written, but also specifically corrected certain of my errors, plus a certain naiveté upon my Tintin history.
Hummm... after some reflection upon how to best integrate our notes & opinions in to a combined, rigorously-fact-checked whole, I'm settling upon it simply being wiser to add-on JM's notes after mine.
Und so, ohne weitere Umschweife: ("and so, without further ado?")
"JM:"
Great post! It's accurate for the most part. I know that Tintin appeared in Tintin magazine until the 80s through reprints and a colourisation of The Blue Lotus (similar to the recent publication). Of course, this is only a minor detail as the last new story to be published was indeed Tintin and the Picaros in 1976. The only other thing, though also a really small detail, is that Le Soir Jeunesse isn't mentioned. The majority of The Crab with the Golden Claws was published in this children's supplement of Le Soir before it ended due to wartime paper shortages.
As to additions, here are a few that I thought of:
You could mention Tintin and the Golden Fleece and Tintin and the Blue Oranges. These are movies but book versions were published by Methuen in the 60s. Of course, they haven't been reprinted since, and aren't in comic format either, so if this is targeted at English speakers it mightn't be that relevant given the difficulty of acquiring them.
When discussing the older versions, it would be a good idea to mention that they often have extra content. Feel free to reference this post of mine if you wish, though the formatting seems to have disappeared with the new Reddit site:
**https://www.reddit.com/r/TheAdventuresofTintin/comments/clz69e/deleted_scenes_from_colour_tintin_albums/
**
If you're targeting French speakers, Hergé: Le Feuilleton Intégral is a really good resource for the serialised versions including cover art of Le Petit Vingtième, Le Soir Jeunesse and Tintin magazine. It is unfortunately incomplete though.
For Quick and Flupke, you could mention the four books that are somewhat available in English. They are Fasten Your Seatbelts, Under Full Sail, Double Trouble and Two of a Kind. The others are unfortunately quite rare in English.
For Tintin and Alph-art, you could mention that the original 1990 album has full size drawings instead of much smaller ones, and is much easier to see details in. That version is much more expensive in English though, as the curent edition has rediscovered pages that the 1990 version lacks.
Thanks so much for creating this post by the way! It's a really good resource for Tintin fans.
Whups, I forgot about Spip et Idéfix! I have adjusted the scoring, above.
So far you are in the lead for this mini-quiz, with 9pts. I scored only 8pts because I can never remember the count / professor's name. (I remember it as "mushroom" in English, but that's not quite right)