How can there be such a discrepancy in interpretations of international law? When Israel is accused of violations, such as over the blockade of Gaza, is it not possible simply to read what's on the books?

Herein lies the problem. Most of international law is not "on the books."

Here's an analogy: Imagine you're at a football game, and you don't really know the rules. You watch these players running around on the field and try to figure out from what they're doing what the rules are. You see one player pushing another, and ask yourself: Is that according to the rules? Is that against the rules? Do the rules even have anything to say about that? And the only people who will give you answers are fans of one team or the other. So naturally, many of them are giving you selective interpretations. In other words, it's very difficult to figure out what international law actually is. Much of it is known as "customary law" - the law that is not written anywhere, but which states do because they think they're legally obliged to do. To figure out what the custom is, you have to see states do it, and figure that the reason they did it is because they think they're supposed to by law.

Precisely because it's hard to know what the actual law is, there is a great reliance on experts. But experts have their ideologies, like everyone else, and many experts mix in their ideology with their expertise. So, you end up with legal opinions that are half politics and half law. And the hotter the legal issue - the more it's something like the Arab-Israeli conflict - the harder it is to get a straight opinion about anything...