Books like Helliconia Summer

Having mostly enjoyed Helliconia Spring I thought I would continue on with the trilogy. (I purchased the omnibus edition.)Events in this book take place several hundred years after the events in 'Spring'. The planet is now at it's closest approach the super giant star that it circles around once every 2,000 years and the temperature has increases so that people in the equatorial regions now live mostly underground. Different to the first book is that all the action takes place over a year or so, so it's a tiny time slice of the ecological changes of the planet. But, people have now been keeping accurate records and hence have a better understanding of what's happening. Most of the story line is very political and SLOW. I had a hard time pushing through to the end, but there was an interesting revelation at the very end. It's also the longest of the three books so hopefully book three will pick up.