Call of Cthulhu was my second love after Traveller.
It had many of the same draws as Traveller, with realistic systems, no classes, levels or mechanical abstractions and it had one major play style difference.
You were not meant to win, just surviving was an achievement.
I never really made the connection CoC had with many other d% games like Rune Quest until years later. CoC to me was an ideal game in itself and so very far away from D & D.
Lots of lovely stuff, more consistent and cross-compatible than the D & D monster has proven over the years (D & D the only game in the hobby large enough to survive 5 complete re-makes). The Orient Express adventure has recently been reprinted, but my (internally) near mint copy is yet to be played. Try playing a 30 year old adventure with current D & D.
D100 games always seem to be in the realism over game abstractions camp, some more than others. CoC straddles the middle ground.
It does not have the gritty granularity of the Runequest/Mythras camp (but could easily have this added), but is more hard core than many other BRP games thematically.
Editions 1 through 6 of CoC are effectively the same. Some rules came and went, some became more complex, a few less, but the mechanical base has stayed the same. Even 7e is basically the same and fairly compatible. Starting at 2e & 3e, 5e is now my poison of choice. Interchangeable with all the others with close ties to 6e, but with some simpler rules like Sanity, 5e was basically state of the art until 7e updated/changed the rules (literally), making all characteristics d% based, instead of d% generating (does not work for me). I have a massive collection of original adventures and period books, even the original Horror on the Orient Express expansion, and a pretty comprehensive Achtung Cthulhu (pulpy) and World War Cthulhu (dark) collections as well, all 5/6e compatible.
Is it a full game?
The basic book is enough. Extra books always add more content, but rarely rules. The several slim Companions are the only real exceptions and they are nice but not needed.
Like WHFRP, there is a lot to fear and plenty for the Gm to work with. A good story teller, someone who has read a few of the Lovecraft or Derleth books, should be able to get years out of it. The entire world in the 1920’s is significantly different to our world, harder and simpler with added monsters, so exploration and adventure are core principals as so much is unknown.
The book does not need to cover every eventuality as again the mechanics are a simple d% system with plenty of flexibility and to help for what is systemically possible. The characters skill set is your best guide. If something is not covered, there is likely a fix somewhere, because the system is in a shared ecosphere, with hundreds of contributors over dozens of games. Unlike D & D, there is an emphasis on non combat skills and story telling.
Monsters are many and often so powerful that your characters may have several encounters with them before they have even a small win. If Cthulhu is not your cup of tea, there are “regular” monsters also, so you could run an entirely conventional 1920’s “Supernatural” style campaign.
Is it relevant?
The game is current, just evolved and regularly comes up in “10 games you must play” lists (editions vary). The support material available is often still current also, such as Achtung Cthulhu, making it a solid choice from new.
Which ever edition you go with, the basic principles are the same. Discover secrets without losing your life or your mind.
Other stuff.
CoC is part of a large, semi cohesive family of games. The BRP (Basic Role Playing) family is deep flexible and almost fully compatible. CoC can be the gateway game or just one form of many. My own personal collection includes many of the old favourites, variations and derivatives although I regret selling off some like Elf Quest and some early Cthulhu books.
Percentage style games are not everyone’s cup of tea, but CoC is a very good fit for this system, so if you want to give a d% game a go, this is probably the most logical place to start, Cthulhu optional.