The 136-page book is well-written as all Greg's books, and is age-appropriate for the audience. It moves the story along well without getting bogged down, or trying to address too many side issues.
The action follows a trio of foursomes as they search out the hidden Great Kanoka disks in Metru Nui. Two Toa, two Matoran journey throughout each section of the giant city, avoiding the Vahki and learning to work with their new powers - and each other. All the while, watching to make sure the Matoran don't slip away unnoticed.
The book provides more detailed descriptions of Metru Nui, enough to really get the feeling of how different this environment is from the island of Mata Nui. The archives of Onu-Metru are a particularly intriguing place, where just about anything can happen... the place is a treasure trove of possibilities. The furnaces of Ta-Metru likewise had a good description, although most of the rest of Metru Nui's suburbs could've used some more description to bring them more to life.
As each pair of Toa follow their (sometimes unwilling) Matoran guides in search of Great Kanoka, it becomes increasingly apparent how different the entire culture of Metru Nui is from Mata Nui. Instead of taking the easy route of recreating Mata Nui in an urban setting, the Bionicle team has really created a totally different feeling for Metru Nui. The characters retain some familiarity, but they are essentially new characters. Whenua, Nokama, Onewa, and Matau as Turaga never seemed to have as much personality as they do as Toa Metru. Vakama and Nuju were more fleshed-out as Turaga, but even their personalities are quite different from what seasoned Bionicle fans know of them in later (storyline-wise) incarnations.
It's impossible not to compare these six characters to Tahu, Kopaka, Gali, and the others. The Toa Metru generally seem more timid in assuming a leadership role, less confident in their abilities to access and control their powers. Probably the largest difference is the fact that Toa Metru were "promoted" from Matoran to Toa, so they have existing relationships with the Matoran the Mata Nui Toa didn't initially have. This may be the reason there's more petty bickering between this bunch - not that Tahu never argued with Kopaka, for example, but there's definitely a difference.
The Matoran are also different in a way. More world-wise, for one, more confident of their place. At the same time, they show a greater range of "human" foibles than their later jungle incarnations. Each of the six Matoran guiding the Toa toward the Kanoka display signs of self-interest overcoming the good of society, which never really happened on Mata Nui.
And the last thing very obviously different: the mysterious ruler of the city, Turaga Dume and his Vahki enforcement squads. The Toa Metru, far from being revered guardians, are forced to sneak and avoid the Vahki squads. The suspense and mystery of Dume leads to the third book of the series, The Darkness Below, which itself presumably leads into the storyline for the upcoming second Bionicle movie.
The latter part of the book is a confrontation with the Morbuzakh, which seemed to have been given short shrift... the climax of the book seemed a tad rushed, especially after most of the book having been given over to the search for the disks. However, most readers should like the confrontation itself and its aftermath.
In all, this is a worthy addition to the Bionicle line of books, and I'm looking forward to the next in the series, which will be available in June.