"Leaf by Niggle" is the closest JRRT ever came to true allegory, and is something of a spiritual autobiography. The tree that Niggle tries to paint but keeps being distracted by details represents his Middle Earth Legendarium, particularly the Silmarillion; Mr. Parish represents his 'secular' responsibilities as a professor, husband, father, citizen, etc. The Journey is, of course Death. The Workhouse is Purgatory. The valley with the tree is the Earthly Paradise, and the land beyond the mountains is Heaven.
"Farmer Giles of Ham" on the surface seems to be a pleasant Midaeval adventure tale, but there are subversive elements to it. In this sort of story one expects the Brave Knight to be the hero; however, in dealing with the dragon the King and his Knights are worse than useless, and the person who is able to take care of the matter is a fat, redheaded farmer who doesn't like tresspassers.
"Smith of Wooton Major" is also semiallegorical, with smithcraft standing in for JRRT's professional obligations as a professor at Oxford (in which his son Christopher followed his father's footsteps, as Smith's son became a blacksmith, too.) Some of the images are odd and disturbing, but beautiful, too.
The miscellaneous poems are great fun. Some, of course, refer to his private mythology; many had appeared in different forms in various magazines and private printings over the years before they were assembled in this anthology. "Princess Mee" is a retelling of the Narcissus story; "The Shadow Bride" is evocative of several old myths, including Persephone, but doesn't quite fit with any of them. "The Hoard", although using tropes from Norse and Celtic mythology is, essentially, an antimaterialist statement--the gold, silver and precious gems that are taken from the earth cause nothing but misery, corrupting everyone who comes to own them; peace comes only when they are returned to the earth in the old King's tomb. "The Sea Bell" and "The Last Ship" are to be read together. Both the speaker in the first poem and Firiel in the second have a vision of another world that stands over against our own--a world of enchantment and beauty in contrast to our mundane existance; the speaker in "The Sea Bell" tries to snach and cling to that other world, and so looses the good of both that world and this, while for Firiel it is enough for her to know that it exists. (Neurotics build castles in the air, as the old saying goes, while psychotics try to live in them.)
The two poems about the Man in the Moon are expansions of two nursery rhymes, allegedly the original forms thereof, and great literary fun. Of the two poems about the trolls, one is from LOTR and the other fits well into it as it refers to places in the Shire. Of the two animal poems, "Oliphaunt" and "The Cat", both are great fun, and the latter is one of the best cat poems I know (more about that below.) "The Mewlips" is a creepy-fun piece, good for a Hallowe'en recitation.
"The Cat", although it seems like a simple little animal poem, is a lot more. The Roman poet Horace said that a poem "begins in delight and ends in wisdom", and this is a perfect example. "The fat cat on the mat. . " contains about the first rhymes a child learns to make, but the poem ends--after a description of various large felines (lions, leopards) ends: "Far now they be/and fierce and free/and tamed is he;/but fat cat/on the mat/kept as a pet/he does not forget." When you put aside considerations of size, long hair or short, striped spotted or solid, a cat is a cat is a cat; the most pampered housecat is a miniature leopard, and the fiercest tiger is a great big kitty.
"Farmer Giles", "The Hobbit" and to a great extent "The Lord of the Rings" are all the stories of small, ordinary people who are placed in extraordinary situations, where they find that they are a lot braver and cleverer than anyone (including themselves) thought they were capable of being. This refers back, I think, to JRRT's WW I experience; he was an officer in a Birmingham-area militia; the men in his company were farmhands, factory workers, shop assistants, schoolteachers, bank clerks, college students--very ordinary young men, thrown into an extraordinary situation; they found themselves doing all sorts of things that they never expected to do--some of them wonderful, many of them horrible, but all of them outside of their normal sense of what should be. The three stories above are all like that. "The Cat" comes into it thus: your ordinary Englishman who might be teaching school or working in a bank or keeping a shop probably has among his ancestors Norman crusader knights, Viking longboatmen, Celtic and Saxon warriors, and perhaps even Roman legionnaires, and the spirit of those ancestors, although deeply buried, under the proper circumstances can come out.