Since writing da book is your main reason for trying Beeminder, have a conversation with us at email@example.com and we’ll get you started off on the right foot. (Tell 'em I sent you!)
In addition to a total-number-of-words goal, many people find it helpful to have another goal to tackle the same challenge from other directions simultaneously, e.g. number of writing sessions per week, or minutes spent writing and/or editing, etc.
And definitely start with a more lenient goal slope than you might think is ideal, especially if you create more than one goal for this.