Amazon Japan released the TOP 10 Comic/Light novel sales ranking for Dec 2007 - May 2008.

Comic

1. Nodame Cantabile vol.20
2. Fullmetal Alchemist vol.19
3. Moyashimon vol.6 Limited Edition
4. ARIA vol.12
5. Mahou Sensei Negima! vol.22
6. Hunter x Hunter vol.25
7. Kyou no Nekomura-san vol.3
8. One Piece vol.49
9. Ookiku Furikabutte vol.9
10. PLUTO vol.5

Light Novel

1. Zero no Tsukaima vol.13
2. Full Metal Panic! Semaru Nick of Time
3. Spice and Wolf vol.1
4. Jinrui ha Suitai Shimashita vol.2
5. Saiunkoku Monogatari Reimei ni Kohaku ha Kirameku
6. Maria-sama ga Miteru Kirakira Mawaru
7. The Melancholy of Haruhi Suzumiya vol.1
8. Toradora! vol.6
9. Toaru Hikoushi heno Tsuioku
10. Boogie Pop Question Chinmoku Pyramid

Source: Amazon Japan (Comic)
Source: Amazon Japan (Light Novel)