Paladins choice

[Taelas]

Indifference to the sufferings of others can be Neutral.

However, taken to an extreme, it may fit part of the PHB description "This person simply has no compassion for others"- which is paired with "and will kill if doing so seems convenient"

Not helping someone when doing so would put you at risk, is easier to justify as Neutral "A neutral person will risk themselves for friends or family, but generally not for strangers"

But when helping someone in trouble won't put you at risk, ignoring their need for help may end up with the character very slowly slipping toward evil (if they ignore others in need a lot)

If you are not helping someone, you are simply not Good, you are not automatically Evil.

Having no compassion does not make you Evil either, unless you combine it with "will kill if doing so is convenient". It is certainly a step closer than someone who does have compassion, but it is still not Evil outright.

It does not matter whether helping someone will put you at risk or not. Choosing to do nothing is Neutral.

"The easiest way for evil to triumph is for good people to do nothing"-

And when evil is triumphing, such "do-nothing" people, may end up slipping toward Neutral and Evil, if they keep doing nothing for long enough.

No. You do not simply become Evil without actually doing Evil. Refusing to do Good DOES NOT make you Evil. Period.

That quote does not concern the D&D alignment system. In most morality systems, if you are not good, you must be evil. In D&D, the default alignment is not Good -- it is Neutral.

What other in-depth alignment sourcebooks are there?

None that I am aware of. That changes nothing regarding their quality.

And which things do you deem them as having "gotten right" and which "gotten wrong"?

Half the examples I see quoted are so misleading that they give people the wrong conclusion. The one you just gave, for instance. It is an Evil act because you kill people who are not actively threatening you simply because it is necessary for you to save your own life. Killing someone who directly threatens you, however, is a pure Neutral act.

Personally I'd say the "context matters" bit, concerning the paladin and the landslide, is bang on.

Context always matters.