could be potassium,.. could be heat, lots of different things cause that, but if they're red like the book says, probably potassium which might be happening because of pH. I noticed in all your pics the soil looks dry. If you're trying to avoid the beginner mistake and not over water, you might be underwatering. First of all don't sweat it,.. it's just your plant talking to you. They look really healthy in the pics so they probably aren't severely damaged. Check pH of not just your watering solution, but the runoff. You can put CFLs really close, but watch the heat. They do give off enought to burn the plants when they're only inches away.
If you haven't already flushed, I would water with a week nut solution that's high in K. Maybe half or quarter strength depending on your soil and how hot that was when you got it. If you haven't changed anything in your water/feeding schedule, then it's probably not nut burn because that would have shown up earlier. Realize that as your plants get larger, they need watering more frequently and can use up the nutes in the soil. You may not need to flush, but simply give them a good water with some K. If you've kept the same watering schedule all along, you might have really dry soil and need to water from the bottom up to make sure you don't have any dry pockets. Or you could water until you see a bit of runoff, then water again a couple hours later until you see the same.
If some leaves have started to curl upwards, then it's more likely a dryness issue. Hydrate them.