The Dragon and The George Series by Gordon R. Dickson

Another series of books that is a lot of fun to read, includes a lot of humor, and characters complete with their flaws and heroic natures is The Dragon and The George and the rest of "The Dragon and The..." ones by Gordon R. Dickson.

The books in this series include:
1976 The Dragon and the George (ISBN: 0345350502, 0586213260)
1990 The Dragon Knight (ISBN: 0312931298, 0812509439, 0586213279)
1992 The Dragon at War (ISBN: 0441756980, 0586214690, 0441166113)
1992 The Dragon on the Border (ISBN: 0441342337, 0441166571, 0586213287)
1994 The Dragon, the Earl, and the Troll (ISBN: 0441000983, 044100282X)
1996 The Dragon and the Djinn (ISBN: 0441002978, 0441004954)
1997 The Dragon and the Gnarly King (ISBN: 0312861575)
1998 The Dragon in Lyonesse (ISBN: 0312861591, 0812562712)
2000 The Dragon and the Fair Maid of Kent (ISBN: 0321861605, 0812562720)

More information: http://www.isfdb.org/cgi-bin/pe.cgi?234

The ISBNs may not be a complete list. It is a good list to start with though: Better than nothing.