Pokemon Diamond and Pearl Adventure(often abbreviated as DPA) is an eight volume manga following the adventures of a Pokemon trainer named Hareta. Hareta is on a quest, given to him by Professor Rowan, to meet the legendary Pokemon Dialga. He is accompanied on his quest by Professor Rowan's assistant, Mitsumi, and the wandering trainer Jun will occasionally join their group as well. Hareta also encounters Team Galactic, and has several conflicts with them as their evil schemes begin to unfold.
DPA is notable for its heavy focus on Team Galactic, to the point where Hareta's encounters with Team Galactic are the driving force behind the plot. Although these encounters often result in serious situations, the manga is, at its heart, a comedy - even the most tense situations can be(and often are) interrupted for a gag. It also tends to be very sappy at times, which flows naturally from Hareta's tendency to assume that everyone is(or can be made into) a friend & his dedication to helping anyone who needs it.
Viz started releasing DPA in English in April 2008, and it just released the final volume in November 2010. It was the first multi-volume manga Viz released since it stopped releasing Pokemon Adventures in 2003(although Viz's translation of Adventures has recently resumed).
As this manga has been finished for several months, I will not require the use of spoiler tags for discussing its plot points in this thread. Be aware of this if you haven't finished reading DPA for yourself!