Sorry to disappoint, but there is one more rebirth book that isn't sold in the cash shop, so there are actually 8 to choose from.
Any book that has the word "Blessed" in the name means it works beyond the 2-month per rebirth limit (The exception is January. In January you can always rebirth, even if you rebirthed in December). For efficiency, ignore books without the word "Blessed" in the name.
That leaves you with 5 cash shop rebirth books...
Any book that says "Initializing Lv." means it will return you to level 1. For efficiency, ignore books with the words "Initializing Lv."
That leaves you with 3 cash shop rebirth books!
The 3 left are 6 Unlimited-Rebirth 70 Lv.Blessed Rebirth Manual, 3 Unlimited-Rebirth 70 Lv. Blessed Rebirth Manual, and Miracle Text of Rebirth.
The 6 Unlimited means you can use it the same month up until 6 rebirths. The 3 Unlimited is 3 rebirths. I'm actually not sure what the Miracle Blessed Text of Rebirth and why it's there.
After you've reached 6 rebirths, you'd move onto the non-cash rebirth book. I forgot what it's called but they're everywhere now. You can get them from Blue Crystal Apples. There is also a quest in the Tower of Ordeals and Tower of Suffering to get a book, and it can be purchased with soul traps and gold (Not worth it). The tower quest can only be completed once, so you can't farm books from the tower stages.
So what's the point of initializing levels? Well that's how the original rebirth books used to work, so they kept them. Maybe you're bored of always being so high level and want to level up by going through the stages again, but that'd be incredibly inefficient. Make a new character instead.
Another reason for going back to level 1 is it also resets all your quests. If you're low on cash and you're confident of your ability, you can redo the tower stages for more rebirth books. But then again those books are very cheap in the market now.
The difference between level 70 and 80 -
If you rebirth at level 70-79 you'll get 2 extra skill points. If you rebirth at 80-89 you get 3 extra skill points. 90-99 is 4 extra skill points. You can buy skill point coupons now anyway, so the most efficient would be to rebirth at level 70, unless you can get to level 80 easily. It's efficient because your exp bonus will increase per rebirth. The time it takes to level 80 at 1 rebirth for that extra skill point, you might've been able to get to 85~90 with multiple rebirths at level 70.
The bonus caps at almost a 1500% exp bonus. I think that's at your 9th rebirth? or 11? I don't remember. It's been a WHILE. (tbh it would be nice if they kept giving incentives to the loyal higher rebirthed players like myself but that makes me feel awfully 1% republican and that scares me)
You might need to confirm with somebody before you go ahead and follow my suggestions. I've only ever used normal rebirth books, 1 month rebirth books, and 1 month lv. 70 rebirth books. The other books came out later on to help newer players catch up to veteran players.