What should I study in preparation for IBOLC?

Get to know FM-7-10 is a help, along with the operation order out of the Ranger Handbook. However, my best advice is to get yourself in the best possible physical condition possible and if you aren't good at it, practice land navigation (without a GPS).
When you get there, read the syllabus for each class and read the sections outlined and be able to answer the sample questions. I do read fairly quick, but I didn't find that keeping up with the reading assignments was that big of a chore. I finished in the top four in the class.
Be ready to live outside a lot. I went though in 1985/1986 and we spent at least half of the course in the woods, which included a really "fun"Ranger Indoctrination Program out at Camp Darby to make sure that those of us that wouldn't get orders to Ranger School had a taste. I was prior enlisted Infantry, so I had been a lot of field time already, but even I thought that we spent a lot of time in the field.

If you are studying the Ranger Handbook that’s good. However, IBOLC will teach more doctrinal Infantry Platoon tactics rather than what the Ranger Handbook offers, but I assure you it will be more crucial to be prepared for what Ranger will be looking for rather than IBOLC. Don’t stress the knowledge portion, you’ll learn what you need to know. You need to focus more on being physically prepared more than anything.

Go into IBOLC in the right frame of mind, ready to learn and motivated and you’ll be just fine, as long as you can carry a ruck!

I am no expert on tactics, by I will tell you something not often mentioned, develop your team-working skills.

You're about done, so obviously you can't do much, but get feedback if you haven't already, from cadre and peers.