Ok, I'm probably not super-qualified to comment on long-term improvement, being a total swim noob, but I've had a lot of success with the TOTAL IMMERSION book.
In 4 weeks, through mostly technique (and some fitness increases), I knocked off 15 seconds/100m, and now can do 8 x 100m at a slow but respectable 1:50 each, with 15 sec rest in between.
A lot of people note that with Total Immersion, they dramatically increase their swim endurance. Most of this is likely because you learn to always swim in a effective "float" position where you waste minimal energy keeping flat in the water, and thus, when you get totally winded, it's a non-issue to just roll into a side or backfloat, which is a central position in the freestyle stroke of TI anyway. (Unfortunately, seems that a lot of folks on TI comment that they don't experience big speed increases until they do sprints.)
I'd highly recommend for you to start by buying Total immersion and trying it out. You'll go from being an obvious rookie in the water to having onlookers with vastly more (but ineffective) swim technique marveling at your seeming efficiency. Plus, it's a lot of fun!