Eglei Tal discussed the halachic parameters of each of the first 12 melachos. He explains the theories involved as well as developing the proper halachic guidelines (according to his rulings).
He has a shorter section where he gives his conclusions about the parameters and rulings, as well as a longer commentary where he traces his conclusions from the gemaras and early commentators.
In addition, in his longer commentary, he compares and contrasts the melachos in light of other mitzvos. So in regards to planting and harvesting, for example, he also discusses the difference between the melacha on Shabbos and during Shmitta. I believe he also discusses the difference between squeezing fruits on Shabbos, and doing so with teruma fruits. Etc.
Thus his work is extremely comprehensive, full of fundamental principles, in addition to being a classic work of halachic guidelines.
It's worth noting that he only wrote on the first 12 melachos. A contemporary edition of his work contains responsa from his work Avnei Nezer dealing other melachos, to make the work more comprehensive.
And Rav Yirmiyahu Kaganoff, a contemporary rav in Jerusalem, has published 3 volumes of his sefer Nimla Tal on the rest of the melachos, in the the style of the Eglei Tal.