Once we have an initial conceptual understanding of generative modelling, we can develop this simple generative model of variable-duration sequences.
The Bayesian view
This is a way of understanding probability that will be most useful for understanding HMMs.
The complete model
Finally we put all those pieces together and we have the complete HMM model, which is actually rather simple.
Reading
Holmes & Holmes – Chapter 9 – Stochastic Modelling
May be helpful as a complement to the essential readings.