What advice would you give to an aspiring author that cannot dedicate full time to writing a book?

Much depends on why you want to write a book and what kind of book it is.  From speaking to other authors and from my own experience publishing several books, it is not something that one can do full-time and have all that time be productive.  My first book took me four years to finish.  The actual writing took less than a year, editing took another six months. Most of the time over those years was developing the material and putting it into practice in a teaching situation.  It started with an outline for a series of talks and the notes I used as references.  I then started to actively blog on the topic to develop a voice and style that might work with the audience I was interested in reaching. As I got feedback on the material and learned what was easily understood and how to express it I decided on a book structure and what would be included.  I spent years doing the background research to find sources that supported my assertions and experience as well as challenged some of my perspectives.

The simple advice I can offer is just start writing, do it often, and get it in front of an audience.  Writing needs to come from having a voice, feeling compelled to to put it out there, doing it in small chunks, and exercise the craft.  The most important is to start by identifying as an author, it makes it easier to think and act like one.