This was a very fun series to read. The books are short (only 100 pages each or so, including pages of beautiful illustrations that add to the overall enjoyment) and each book ends on a bit of a cliffhanger, so you really need to read the whole series to enjoy it.
Three siblings (twin boys and their older sister) move to a relative's old house with their mom and discover their great-great-uncle's book about goblins, dwarves, brownies, elves, etc. - and these creatures live in the land but are invisible to the naked human eye! They make mischief around the house and the area, and soon the siblings find themselves drawn into a battle in this world because of the book's existence and their possession of it.
The three kids are very easy to like. They're pretty typical, each with their own interest, and occasionally bicker between themselves in an amusing way. I continually wanted to root for them and to discover what they'd get into next. Plus, as I mentioned earlier, the illustrations are excellent as well.
The only thing that surprised me about this series was that it's supposed to be for ages 7+ but had a bit of violence in it and the word "crap" was used a number of times. So, if you're planning to read this with kids, just be aware.
Overall, an enjoyable series. The plot was interesting, the characters were good, there were both funny and exciting moments, and the writing was excellent. Now I'm looking forward to watching the movie and seeing how it measures up...