The series does an excellent job in fitting in the most important things from the book to get the feeling of the setting and the characters.
Of course ideally the series adaptation would be more than twice as long with 10 times the budget to really put the books to justice but in the real world, with realistic time and money, they really did a most excellent job with this series and even the movies that are a bit shorter.
Loved the books, loved the series. If there were anything I would like to add to the series is how Arn and his family and friends helped build Sweden, after his return. How he brought along war knowledge and builders and boosted the economy along with his brother and his son.
But maybe there is this room for these adaptations in the future.