Most of the time the books are saying that because when you feed, you should also be checking up on the rabbit. Seeing it move and get excited for food. If a normally excited rabbit is at the back of the cage hunched over, there could be a problem.
A lot can happen to a rabbit in the 24hr period between meals. But if you check up on your rabbits several times per day like I do, feeding twice a day is not necessary. If you only look, pet and play with the rabbit once a day at mealtime, and that's all you have time for, then maybe increase the feedings to two meals.
I feed my rabbits half a cup of feed and a large handful of hay per day. For most of the rabbits, this will last all day, especially the hay. For some, they'll have it finished by bedtime, but they got their portion and need to wait until breakfast.
A good way to judge if your rabbit is getting enough to eat is if they look hungry in the morning. If they're practically breaking down the door and scarfing their food, they probably need more or to be fed twice a day so they don't eat it all in one sitting. If they are excited for their food but pause between bites and take their face out of their food dish, they're getting enough. If they are never excited for their food and lazy, you could probably cut back on how much you're feeding them.