do you want to do the series and race well or just do it for fun and train hard for the spring race?

if the series is the most important make the last race your "A" race and get a trainign plan or coach to develop a plan around it and use the other races as hard training days

if the spring race is the "A" race then train/race  the series and build your training plan around that race

trinewbies.com, beginnertriathlete.com, trifuel.com and slowtwitch all have generic training plans that you can use or tweak

where is the race series if you don't mind me asking?

If you don't join a tri club, definitely try to find a training partner.  I really recommend the book "Fitness Cross-Country Skiing" by Steven Gaskill.  Obviously it is designed for skiers, but it breaks your year into four training segments, including one for your competitive season, and gives you specific daily workouts and shows how to space them over a month and integrate them with appropriate recovery during race season.  It is very easy to adapt the workouts to swimming, biking, and running, because each one gives very specific heart rate targets, and includes an alternative to skiing (such as biking, running or swimming ).  The book also include excellent methods for testing your fitness, so you can gauge your progress.