An interesting point about this Robert Heinlein book is that it presents a very comprehensive guide for setting up a secret organisation (the people in the book are being oppressed by the evil colonial moon authorities). The reason that it is particularly useful is that the book was written in the electronic era, and because Heinlein's writing was so forward-looking the communication, political climate and infrastructure he describes are very similar to the present day in many ways.
The structure he presents is basically a human trinary tree. The ruling triumvirate are a cell. Each of them is also a link to another cell on the next rung down, but noone in the top cell knows who the other two have recruited. Each of these subsequent cells then has the same pattern, with noone knowing anyone except their own cell, their senior contact and their own recruits.
Heinlein used a clever method for identifying the rank of an otherwise unknown member. The members of the first cell gave themselves aliases starting with the letter A. The people in the next level down had names starting with B and so on. This system allows for a very large number of people to be in the organisation - 3^26 or thereabouts - and for them to communicate with a maximum of 25 steps between themselves and the top of the tree. It also allows those at the top to estimate how many people have been recruited depending on which letter of the alphabet the lowest-ranked communications come from.
The key to the security of the structure is that there are no horizontal links between cells on the same level. Thus, if one person gets caught then under interrogation they themselves can only reveal at most six other members' identities: the remaining members of their cell, the person who recruited them and their own recruits. In the time between their arrest and this information being revealed to big brother the other cell members have a window of opportunity to sever ties, report the breach, destroy records etc. etc. and thereby minimise damage.
A final relevant element of the organisation is the existence of Mike, the Moon's supercomputer. He controls, amongst other things, all electronic communication networks. The reason that this is particularly pertinent is that, while we don't have intelligent computers today we do have the Internet. So whereas in the novel Mike would have protected communications from eavesdroppers in reality we can use encryption on the Internet. No wonder then that the US government is so keen to control the sale of strong encryption and to spy on e-mail using the carnivore and omnivore systems.
On a purely literary note, The Moon Is A Harsh Mistress flies in the face of popular perceptions of Heinlein by presenting a pro-libertarian agenda, in contrast to the right-wing political antics of Starship Troopers. It is also notable for one of the most coherent presentations of a fictional future language in any science fiction novel ever - it is more believable than Orwell's Newspeak, for instance. Basically, Heinlein predicts that 'meaningless' words like 'a' and 'the' would be dropped from the popular vernacular. After a few pages it becomes disturbingly natural to read.