I have the same book, and using that technique does tend to lead to an ugly build up of paint. I use a similar system to allan above, I Airbrush a coat of desert yellow mixed with flesh, the exact ratio is not important, I tend to go for a pinker look for paler skin, and a yellower look for darker skin. I use a soft brush to remove the oil paint rather than a cloth, and once I have done this I leave the face for about half an hour, then using a mixture of matt white enamel and the original burnt sienna/naples yellow/yellow ochre (depending on your preferance) base coat, gently work in some highlights to the base coat, on the areas like the cheek bones, bridge of nose, top lip, forehead etc. I leave this another few minutes (now you've mixed the white enamel this layer will dry a little faster) then using another clean DRY brush(NO thinner at all, otherwise all the paint will come away), wipe off the highlight paint leaving ony a thin coating. Make sure you do not completely remove all the highlight paint or blend it completely into the surrounding base coat, other wise you may as well not have bothered. The trick is to gradually build up the layers of paint, applying it more as a stain than a coat of paint. You can now add some extreme highlights with almost pure white, just to the very tip of the nose, above the eyes, highest points on the cheek bones, blend thiis into the surounding areas carefully, but not so much that they dissapear.
Now for the shadows, using burnt sienna/raw uber mix, carefully work in a few shadows into the darkest areas. You can carefully add a little shading around the beard area to simulate 5 o'clock shadow.
carefully work a little pink to the cheeks, you can dry brush this afterwards or work it into the wet oils, whichever you prefer, and add a little dark read/brown colour to the lips, but don't over do it, you don't want them looking like they have lipstick on!
to paint the eyes work white enamel into the eyes, the oil paint already there will make it a bit pink, that is good, pure white is too bright. once this is dry use balck or dark blue and paint a dot in the centre, but make sure that the balck meats the top and bottom eyelid otherwise they will look pop eyed.
hope this helps
Phil