I am a complete DT newbie. The Astonishingly is my first DT album and has completely blown me away. Never heard anything like it before, I've taken the time to read the story and follow the lyrics, and wow! The musicianship is amazing and what I really love about it is that despite its length, it is accessible with loads of memorable and catchy stuff throughout. Can't stop playing it.
I come from the melodic side of rock/prog...earlier Genesis, Marrillion, Jethro Tull, Spocks Beard, Pendragon. I get the impression that The Astonishing is a one off, a rock opera/ musical theater thing. I'm a sucker for a good melody/power ballad, but also love raw rock power.
Which album should be my second purchase? Btw, can't believe I have missed this band for so many years, really excited to discover more!
Welcome aboard
If you liked the concept album theme check out Scenes from a Memory (which is head and shoulders better than The Astonishing in my opinion.
Truthfully any album is a great choice.
My dad got into them through Awake (which is another great record and shows off a more metal side)
Summary- Scenes or Awake would be great choices but you can't go wrong with any album.
Probably wouldn't start off with Train of Thought (Dream Theater was trying to be Metallica and Megadeth on this one) or When Dream and Day Unite because they had a different singer.
Even there weaker ones (Black Clouds, Systematic Chaos, Self Titled) are great