The document "Wood gas as engine fuel" is a must read for anyone wishing to build a wood gasifier. It concentrates on downdraught gasification, which is best type of gasifier for fuel grade wood gas.
The document by the Food and Agriculture Organisation of the United Nations (FAO) covers all aspects of building gasifiers for fuelling engines. The document can be found on the FAO website at http://www.fao.org/docrep/t0512e/t0512e00.htm.
A PDF document can also be found at ftp://ftp.fao.org/docrep/fao/t0512e/t0512e00.pdf