I'll second this (though I've only read the first thus far). I don't know that I'd consider it especially hard SciFi but it's far from a space opera. I recall feeling like the justification for the creation of the arachnid race was a bit hand-wavey, but the level of thought put into their society more than made up for the required suspension of disbelief. Definitely one of my favorite books.
For something similar I'd also recommend Dragon's Egg by Robert L. Forward. It's about the discovery of intelligent life on a neutron star, who develop at a rate exponentially faster than humanity. Also not super hard SciFi, but a great exploration into truly alien life.