It's not a problem. Getting the book is your discretion, honestly there's no guarantee that you'll get selected. If you want to wait until you do to get the book, that is fine as you'll have a little bit of notice of the test date.
Typically this is the case with the public application. I have heard that they get a lot of applicants and the FAA started advertising the application on websites like Monster, Hot Jobs, Faceboo, My Space and others, along with radio and TV spots in select regions.
As for the percentage that go on, there's really no exact number. However, a class I was in during school did speculate this:
We assumed that 10,000 people applied for the position and roughly half of them were eligible for the AT-SAT. ($800 x 5,000 = $4,000,000)
Of those who took the AT-SAT, roughly half of them were qualified for the position (2,500)
Those that went on to attend the PEPC, half of them were qualified enough to be considered for positions (1,250)
Most of those left either turned down positions/facilities for others or had lost desire to actually continue since the process is so long... we assumed 1/4 of the 1,250 (1,250-312 = 938)
Those that were left went on to OKC and at least half either dropped out or gave up (469)
Those went on to their facilities and around 75% did not make it through training at their facility (352 lost... 469-352= 117)
Again, we speculated this... none of this is likely accurate but gives us an idea that most who apply don't make it. However, keep in mind that there are many that apply with some knowledge or experience in the field and those people have a greater chance of getting through.