I'm struggling to find a good resource that explains how roll, pitch, yaw angles are calculated from the X, Y, Z measurements of an accelerometer. I came across this document but the explanation on page 9 is still very sparse. In particular, I'm unable to understand the following 2 lines:
The accelerometer vector lies on the surface of a sphere with radius 1g. It is not therefore possible to solve for three unique values of the roll φ, pitch θ and yaw ψ angles.
Suggestion for a good book or web resource would be much appreciated.