My favorite book review site, Bitten By Books, has reviewed all 5 of the Rowan of the Wood books. Here is an excerpt from their final review for Spirit of the Otherworld: 

The end of a series is always bittersweet. While it is satisfying to finally have every mystery solved, it is also like saying goodbye to good friends. The members of the Freak Squad definitely feel like friends at this point. Spirit of the Otherworld is a fitting end to what has been a mostly strong, refreshingly different young adult series. In a market saturated with vampire stories, the Rowan of the Wood series gives this genre a new twist, satisfyingly combining vampires with Celtic mythology. While there may have been a few minor bumps along the journey, this is definitely a series I would recommend for readers of all ages.

Read the entire review on their site.