[GeoffR]

I see VIP membership can be bought by redeeming 4000 super points. So if you spend $200 per year you would, at a minimum, earn enough points to buy the next year's membership.

Looking at it that way, the first 4000 super points you earn are worth $2.50 per 1000, or equivalent to a discount of 2.5% on total spend. (Assuming VIP membership is not discountable.) Points above the first 4000 might be worth less, depending on whether the books you redeem them for are discountable or not.

Edit: So if you spend more than $400/year it is worth joining the VIP scheme regardless of what sort of books you buy. But if all the books you buy are eligible for the extra 10% VIP discount then it would be worth joining if you spend as little as $81.70/year, since you would save at least $8.17 and earn an extra 735 super points which are worth $1.83. (This is not counting the other things you get with VIP membership: free book, bonus days, promotions.)

But if you spend between $81.70/year and $400/year then it will depend on how many of the books you buy are eligible for VIP discount and how much the free book is worth to you, and if you spend less than $81.70/year, then it is probably not worth considering VIP membership unless one of the free books makes it worthwhile.

(Edit2: For those in the UK, divide $ amounts by 2 since membership costs £5 instead of $10. Note that the above doesn't include the value of the bonus days or VIP promotions, as I have no idea yet what they will involve.)