For Oracle9i DBA, it's required to take at least one course from Oracle university. It's pretty expensive. I paid $1250 for an online course and $2500 for in-class course. One way to get around that is to take the Oracle8i DBA cert and then take upgrade exam to 9i. One draw back is that you have take 5 exams for 8i and plus one upgrade exam, compare to just 4 exams for 9i. You still save money by doing 8i first.
I bought bundle 9i books from Sybex. They were pretty good books over all, except for the 3rd book, and they'll prepare you for the cert exams.
I recommend different book for 3rd exam(Fundamental II) because Sybex book for that exam is horrible. I heard Oracle press book for that exam is pretty decent. Also, try download the Oracle9i database from them(free for learning) and do hands-on practice.