New Essays On Human Understanding Book

Ostensibly a 'dialogue', Leibniz basically gives a line by line, paragraph by paragraph summary of Locke's "Essay Concerning Human Understanding" with his own responses in between, sometimes outright quoting blocks of the original text. that's an advantage really: you're basically reading two books in one. but it does make it more difficult to interpret the text as a unified work that develops a single argument representative of Leibniz's mature philosophy.
There are some key themes Leibniz latches onto in Locke's text, which is great because it helps clarify what was at stake in the book to begin with: are there innate ideas? what is the contribution of the intellect (if any) to the formation of ideas and concepts? are there real essences, or only nominal ones? how do words attach to ideas?

Leibniz also goes into some appropriately batshit territory: if there are aliens on the moon, and they appear to have rationality, should we baptise them? (it's a tricky one because jesus only died for the sins of mankind ' leibniz says we should baptise them provisionally, just in case).
I enjoyed Leibniz's treatment of molyneux's problem'he's just about the only major philosopher from that period to figure out that since blind people can do geometry, then spatial ideas probably aren't just visual in character.
In fact, these little asides are probably what's most memorable about the text as a whole, and they do give you a good idea of some key Leibnizian themes: yes we should baptise aliens, because they possess rational souls (which are the essence of humanity); of course blind experience is transferable to vision, because clearly understood ideas reside in the intellect, not the sense organs.
but there's plenty of boring shit in here, like Leibniz's speculative etymology and his long theological ramblings about whether unborn children go to heaven etc etc
overall i give it four 'monads' out of five !! haha