<Tr> <Td> 2006 </Td> </Tr> <Tr> <Td> 2007 </Td> <Td> Halo 3 </Td> </Tr> <Tr> <Td> 2008 </Td> </Tr> <Tr> <Td> 2009 </Td> <Td> Halo Wars </Td> </Tr>

How many books are in the halo series