I'm sure i'm not the only one who has problems re-introducing my characters and concepts in book two. I find repetition to be a sin, sometimes a necessary one. I think I am guilty of over describing to compensate.
It has been such a long time since I posted here!
How do you re-introduce your characters and concepts in sequels?
Try and see your work as a reader, not a writer. I re-read my first book a couple of times (quite enjoyed it, too!) and then began book 2. That helped me set the level of detail that worked - surprising how little was needed in some areas, and how much was needed in others. Admittedly, I had a (very) broad synopsis of books 2 and 3 when I'd started on book 1, which did make it easier.
Are your books definitely serial only, or do they stand alone? A reader may come across book two without having read book one, so you have to give them whatever info they need to access the characters and the place, but without being too heavy-handed about it.
You could show signs of aging. How younger characters have grown. Whether characters have changed their appearances. For example the colour of their hair. I would use dialogue to show this.