To set the book-type on +R SL or +RW media you will need to use a modified firmware such as Liggy and Dees, or a firmware from a NEC-3520A knock-off which supports this because the standard NEC firmware does not. Using either will void your warranty so make sure that you have upgraded to the latest stock firmware and thoroughly tested your drive to make sure that your drive is in good working condition while you still have your warranty. It is a good idea to back up your firmware before flashing with a modified one.
It's possible that only +R book-typing is now supported which is the most important. Wasn't the +RW support dropped because of problems? You may need to look this up because I'm a little rusty on this and I don't have a 3520A, but a 3500A and have been using up a supply of -R's that were on sale. Anyway, book-typing a +RW to DVD-ROM will sometimes cause an otherwise working disc to no longer play in some commercial players. Therefore it must be done on a case by case basis because you don't get the same compatibility benefit as with the +R disc. If you don't use or need the +RW book-typing, you can just set the drive to book-type +R DL and SL to DVD-ROM and forget it without any further changes.