Besides the Brihadaranyaka and the Chandogya Upanishads the Maitri Upanishad is the longest of the principle Upanishads. Consequently, it is able to cover a great number of philosophical and religious topics common to the Upanishads. Of course, central to the work is a discussion of the soul in which the individual soul is described as an extension or manifestation of the universal soul.
A significant amount of text is devoted to describing the relationship of the soul to the body. The soul is characterized as the "driver" of the otherwise unintelligent body and the body is further described in most unpleasant terms, "It comes forth through the urinary opening. It is built up with bones; smeared over with flesh; covered with skin; filled full with feces, urine, bile, phlegm, marrow, fat, grease, and also with many diseases, like a treasure-house with wealth." The point, of course, is to frame one's perception of the body (or the material world in general) in a way that invites escape into a purified world of the unembodied, undifferentiated universal soul. This escape is described in the fate of the soul after death, specifically not reincarnation, but release from the cycle of rebirth and repeated death.
A large section of the Maitri Upanishad is devoted to yoga -- the practice of austerity and meditation, including breath control, but it also highlights other topics within the Upanishadic tradition: the significance of the word "Om," the place of sacrifices in efforts toward liberation, and the symbolism of the sun as Brahma. Among the more intriguing topics is food. Much is said here and in other Upanishads about food that is sometimes puzzling. Sometimes it is discussed metaphorically and sometimes literally. In the Chandogya Upanishad it is used to mean earth, one of three divinities: fire, water, and earth. Elsewhere, it appears to mean whatever is necessary to support or nourish something; hence fuel for fire is food. Food is also used to connect all things in the material and non-material world, creating a kind of ecosystem:
From food, verily, creatures are produced,
Whatsoever [creatures] dwell on the earth.
Moreover by food, in truth, they live.
Moreover into it also they finally pass.
The Upanishad ends with several sections warning against false teachers, ignorance, perverted doctrines, and devilish, false, non-Vedic doctrines.
In all, the Maitri Upanishad offers a fairly comprehensive presentation of many of the central doctrines of the Upanishads and should be one's short reading list to understand the Upanishads.