While I have seen a number of basic triathlon plans that do not incorporate strength training (focusing just on the swim, bike, and run portions), I think all of the comprehensive plans that I have seen in book format do incorporate strength training. However, the strength portion is usually found in a different part of the book and not actual built into sample training plans. Kind of funny really, if it is that important (and I think it is), and one of the huge aspects of long-distance triathlon training is strength, it is a bit odd that they do not include it.
If I recall correctly, my first year training for half-iron distance races, I cut back to once per week for strength training at some point, and maybe cut it out completely as I got closer to race day and was pushing to keep up with the swim, bike, and run training. At that time I was participating in a strength class at the gym.
I currently do not belong to a gym and have not for a number of years. For the last few years I have been doing a workout that I can do from home. I think it helps to keep basic strength and really enjoy it. Someday I would certainly like to be more focused on strength and to work with a trainer in a gym, but that is not in the cards in my life right now.
The plan that I am following currently, a workout I do twice a week (most of the time) is the Ironstrength workout designed by Dr. Jordan Metzl. It appears to be available for free online currently, or you can purchase the DVD for home use. I received a printout when I ordered the DVD that I followed instead of the video version.
(And I will admit that I still do not do the full pyramid of burpees, even after years of working this program. I think that is a good sign that it is difficult enough to push us for awhile…or maybe I just do not push myself enough.)