Alongside the art of making decent equipment, the quality of manuals has withered the last few decades.
Not only the digitalisation of manuals (try to fix modem problems with a manual only available on-line) but the shortening or unorganised manner has driven a growing number of people to support media.
From going to the local fixer, the endless waits with call centres, to waiting for day hoping to get a return to your support mail.That mail will likely be a standard mail telling you to hold on for some more days.
Manuals can be either be too easy, having only a page of main instruction on how to turn the thing on and off. Or either you get an encyclopaedia that gives you everything but nothing at the same time.
Mind you, the digital manual has it’s merits, especially the search function.
The manual should be proof-read by outsiders, who got no affinity with the subject. In no other way can every one understand how the product works. No assumption of one’s knowledge may be made. The brain’s power is great, but cannot encompass the vast library of knowledge of the world. Unless you are like Kim Peek.
Proof-reading is a key to have an understandable and useful manual. There may be no way to cause confusion upon the user. Especially contradictions are deadly. If a certain part of the product needs prior knowledge, refer to a good and freely available source if it cannot be included in the manual as an appendix.
Be aware not to confuse the user with too many footnotes, it will lead him away from the subject.
The manual reflects the quality of your product. a flimsy or a badly constructed manual will reduce the value of your product and your brand image severely. The human being is an inventive, proud animal and wants to solve their problems as much as they can by themselves. They don’t want to spend hours getting frustrated to solve their problems and for not having any support of the people who made the product.