Format
The manga was serialized in the magazine Nakayoshi in Japan, and has been published in America in the format of four graphic novels (which are unflipped, meaning they are meant to be read in the original Japanese right-left format instead of the usual English left-right pattern), each of which has several chapters. The chapters themselves are printed in the usual (for manga) black and white.
There were color art pages (common in manga serials) featured in the manga's original release in Japan; these pages are also featured in the American graphic novel release of the series, but not in the original color.
Most of the chapters, while sequential and technically continuing several main plots and subplots (or to be more accurate, character development arcs and the central sometimes-background plots involving the mysterious character Sirius and the disappearance of Riri's mother), are what are also known as "stand alones", meaning the main plot (or mystery, in this case) of each episode is not specifically connected with any of the other episodes, and can be read and understood without having previously read the series. A couple of the mysteries are two-part episodes, but the rest are mostly self-contained.
Read more about this topic: Zodiac P.I.