It really depends on the author. Some writers deliberately refrain from promoting their material allowing people to learn about them via word of mouth (I'm guilty of this); Some prefer to sell only a limited number of units (typically less than 500 with many being limited to as few as 50 copies). However, there are those that put books out and in a week have enough to put a nice down-payment on a new home or buy a car, etc.
The other factor you MUST keep in mind is the fact that every 12 year old with a computer that's "studied" magic for more than a year believes he/she is ready to write a book and sell it on line via Lulu, etc. In other words, the market is way over saturated and in order to sell a book you need to build value into it. The first step to this end would be creating name recognition; submit ideas and routines to the various on-line publications as well as the classics like Genii, MUM, MAGIC, Linking Ring, etc. Just get your name out there!
After a year or two you'll be established enough to actually move a decent amount of product with that first book. . . but research things first. Talk to actual publishers and old guys that have an actual track record when it comes to knowing if or not what you produce is worth the hassle of putting a book together. Too, don't be a cheap skate! If you write a book put at least a dozen or so effects. . . good effects in the tome along with a solid sense of structure. Price it in a manner that is fair and "humble" unless you have the kind of accolades around the material that suggests that you can pull a Jerome Finley and charge $1,000.00 for an eBook.
Take your time and think about what you're doing, why and if or not you've developed the level of reputation that will insure sells and support the price.