One of the oldest disputes about the Mencius is whether Mengzi himself actually wrote it; over the many centuries, both sides have been argued so extensively by the many Confucian scholars and commentators who have discussed the issue that there is little for modern scholars to add to the debate.
In the Chinese, each book of the Mengzi has a title, but it is purely a convenience for reference rather than anything informative: it comes from the first sentence of the book and is usually the name of the person with whom Mencius happens to be talking at the beginning of the book.
You can read the Mencius online at the Chinese Texts Project, in James Legge's translation. As for myself, I will be using D. C. Lau's translation from Penguin, just because it happens to be the copy I have on my shelves.
Book I.A (Liang Hui Wang I)
Book I is unusual in the context of the rest of the work in that it seems to be arranged in at least a roughly chronological order; as such it is a major source for the life of Mencius himself.
We begin with Mencius interacting with King Hui of Liang (or Wei). King Hui himself gives a historical rundown of his very troubled career at I.A.5; his first comment to Mencius in I.A.1, asking how Mencius can profit his state, should be read in that context. Lau in his introduction gives the key dates for events King Hui mentions:
341 BC -- Defeat by the Kingdom of Qi
This was followed by a period of twenty years in which he suffered repeated defeat by the Kingdom of Qin.
323 BC -- Defeat by the Kingdom of Chu.
319 BC -- Death of King Hui
Thus King Hui is at the end of a disastrous career, in the last few years of his life, and is attempting to get himself out of a terrible situation. Master Meng points firmly to what he sees as the problem: it is precisely this thinking in terms of profit rather than in terms of humanity (ren) and rightness (yi). If everyone thinks of profit alone, nobody is ever satisfied, and nobody actually works with anybody else for the good of parents or rulers. In I.A.2, King Hui is in the midst of his wealth and asks if it can be enjoyed by worthy people; Mencius replies that it is only worthy people who actually enjoy them, because delight is something shared with others. King Hui insists in I.A.3 that he has done well and has just had bad luck, but Mencius is not impressed: his misfortune is actually the effect of his own actions. Perhaps he has done better than other kings, but that's not a high standard. A true king regulates things so that his people have plenty to eat, and that everyone has a little something to contribute to that plenty, and that they are well educated so as to fulfill their responsibilities; if that were the case, he would not have problems:
When people die, you simply say, "It is none of my doing. It is the fault of the harvest." In what way is that different from killing a man by running him through, while saying all the time, "It is none of my doing. It is the fault of the weapon."
When King Hui asks in I.A.5 how he can wipe away the shame of his many defeats, Mencius insists again on the importance of benevolence. This is not an abstract ideal. Other kings go around harassing their people, destroying their ability to survive, miseducating them. A king who actually makes sure his people can eat and do their work properly and who teaches people to uphold their responsibilities to their families, creates a wealthy and tightly-knit people who will be able to accomplish things that could not otherwise be accomplished.
After King Hui's death, he was succeeded by King Xiang, and Mencius seems to have been so thoroughly unimpressed by him (I.A.6) that he left for another state. The next conversation (I.A.7) is with King Xuan of Qi, who had just recently ascended the throne. The discussion is quite long because King Xuan seems to be extraordinarily reasonable and willing to discuss his own mistakes and failings. Mencius encourages him to recognize the fact that his ambition to make his state and reign great are not achieved by tumultuous wars against foreign enemies 