As others have said, you can work in whichever direction you prefer.   As the book is laid out "from the oldest ancestor down" and you think it contains 100-200 people I assume a lot of these people are not direct ancestors of yours but great-great-uncles/aunts and cousins several times removed.  So whichever end you start from there will be some 'diversions' when you record branches from which you are not directly descended.

Also, it sounds as if the book deals with just one of your ancestral lines, starting with the earliest known ancestor in that line.  So it might cover (at a guess) 8 generations in that line. But don't forget that these are not "all your ancestors".  If you count yourself as generation 1 you had 128 direct ancestral branches 7 generations further back (assuming there was no intermarriage between cousins) so there are a lot more ancestors to discover!

Lorna