Calibre is too powerfull and versatile for its own good, and the mere mortals can't handle it!
There
already a simplified manual: the "Calibre: Quick Start Guide", which is automatically added to your library when you first start the program.
The problem here is that few people read manuals, even simplified version, myself included. There are many reasons for this, and unfortunately, it is an unsolvable problem because, well, the mear fact that few people read manuals, even simplified version.
(and that the same reason that people don't read tooltips (if you even know that their exists))
A manual is verbose by nature. You don't know what you're really looking for, or even if it's hidden under another name, until you stumble across it on a page, whether the feature exists or whether you missed it, even with a table of contents. And even if you know it's there, in that specific section, reading and understanding it is an intellectual effort... and human beings are profoundly lazy creatures, so unless you have a good reason to do so, you rarely read the manual.
It is also much more effective for people to learn how to use something by experimenting on their own, or if they get stuck, asking for help from people who know more about it to guide them step-by-step, by dynamically adapting the language to their actual understanding to maximize the learning.
When a manual, by is static nature, if you don't understand what you're reading, you're screwed. If you don't get it, you don't get it.
At this end, the manual is more a support to help the helpful user to guide the new user, rather that a direct solution.
So, most of things need to be do inside of the GUI as much as possible to facilitate the experience of new users by optimizing the interface, guiding the user so that their experience is as smooth and comfortable as possible. But UIX is a field in own for a good reason, and is one of the areas where FOSS/Open-source software often faill.
On this note, Calibre is relatively good because it had plenty of time to refine it thanks to lots of feedback from users. Improvable probaly, but unless a UIX expert conducts a comprehensive review, we proabaly don't able move very more, and mostly by little step.