The biggest one for me was Richard Wilhelm's translation (with annotations -- the annotations are really important) of the Tao Te Ching. Following that, the same translator's work on the I Ching. (Sorry for the potentially weird transliteration -- I never know what to do for Chinese!)
My own interpretation is that while we can say "I think therefore I am" you can't really prove anything else. The tao takes it some small step further: There exists you, but there also exists everything which isn't you (even if that's nothing). So, in other words the existence of A implies the existence of not-A, even if that's the empty set. Between A and not-A there is a boundary where you transition from A to not-A. In other words, by implying the existence of one thing, you imply the existence of 3 things, even if 2 of them are the empty set.
This turns out to be really handy. If you say "tall", it implies "short" (not-tall) and it even implies that there is a point between "tall" and "short" where it transitions from one to the other. This is actually a tautology, based on the definitions of the words, but it can be used as a base of reasoning. The classic example is that a cup can be empty or it can be full. A cup that is always empty is not useful, and neither is a cup that is always full (for the normal definition of "useful" when applied to a "cup"). It has to transition between empty and full for it to be "useful". Similarly, a spoke in a wheel is defined as much by the places where it isn't as it is by the place where it is. You can't have a spoke without a space and when you think about spokes, you should also think about the spaces between the spokes because they are just as important. Or if you don't want criminals, one easy way of accomplishing it is by getting rid of laws -- no laws, no criminals. More laws, more criminals. This is just reasoning based on the tautological definitions of words, but it can lead you to pretty interesting insights.
The I Ching is interesting because it documents 64 states that you could possibly be in. It then describes all the transitions between the states. There are consequences for moving from one state to another. The consequences are described in those transitions. It is surprisingly apropos! Though it was supposed to have been written by the Yellow Emperor thousands of years ago, in reality I think it's been tweaked over and over again for thousands of years and the 4096 different possible transitions have just been worked out by exhaustion.
Many people think of it as a fortune telling thing and you can roll dice (or draw straws) to pick a starting condition and the transitions, but my impression is that this was originally a study mechanism. In fact, it's not a book you can easily read through. Randomly selecting a starting condition and transitions allows you to study one thing at a time. I still haven't gotten through everything :-) Basically, I think of the I Ching as design patterns for life. I'm here. I want to go there. What is the best way to go about it and what are the likely consequences of those actions? Very useful.