Mindfulness of Breathing is a unique meditation technique in that it fulfills all the pre-requisites for enligthenment, if we carry out the 16 steps.
I have found Ajahn Brahms interpretation of the technique much more intimate and less scholarly, and easier to understand on a subjective, personal, intuitive level as opposed to Ven. Vimalaramsi and Ajahn Buddhadasa's meditation guides.
There is another thread here on his ebooks:viewtopic.php?f=17&t=199
"Sensitive to rapture" - in my limited understanding is that one actually feels it, as one drops each of the 5 senses one by one (leaving the mind sense), this is particularly blissful, as the mind lets go of the world of the senses.
You should have "calmed the breath" beforehand (naturally, without gripping or force; this is not pranayama!) the rough texture of the breath at the point of contact should have changed to a smoother texture kind of like rubbing denim on your upper lip to rubbing silk on your upper lip, eventually, it gets very fine that perception of in-breath and out-breath, long and short breath, will.... disappear!
Be sensitive to pleasant feelings at this point, relax, if the pleasant feelings don't come, don't worry, it will come, at this point, the purity of your mind and sila is important, if it doesn't, try to cultivate wholesome mind states through the day.
Or you haven't sufficiently cultivated the first 4 steps. So you have to let the steps gradually unfold, we can't rush it, strong buildings are built on solid foundations, if we rush through the first 4 steps, it is like building a house of cards.
Most importantly, go on retreat, consult your teacher in real life to learn the technique,; everything will happen naturally, you can't force it, if your mind states aren't good to start off with in the beginning of the sit, learn to do chanting, metta, and the 10 anusatis (reflections), double up on your efforts to protect your sila and wholesome mind states throughout the day.
All the best.