The book was originally a publishing concoction consisting of only two items, short story 'Leaf by Niggle' and essay 'On Fairy-Stories'; both have since become readily available in other editions. But even if you already have the texts in your collection, know that 'Tree and Leaf' has sprouted new branches since its first publication in 1964, as is only fitting. First, in 1988, Tolkien's poetic dialogue 'Mythopoeia' was added; this new 2001 edition also includes 'The Homecoming of Beorhtnoth' - the only sequel to another author's work by JRRT that I know of (albeit one nameless, and dead for a milennium). The four works now form a strong whole: the essay lays out the groundwork for virtually all of Tolkien's fiction and sheds much light on the value system underlining his creative choices. This is literary theory of a sort that is almost extinct in university courses nowadays - and feels like a breath of fresh inspiration after so many postmodern dead ends, frankly. 'Mythopoeia' brings this argument with modernity out in a style reminiscent of classical dialectics (in verse and quite amusing to boot); but then 'Leaf by Niggle' explodes in a flash of what the book has previously only talked about: it is the real thing, one of Tolkien's most poignant works, and the sheer concentration of emotion in it rivals his best mythological stories. 'Beorhtnoth' now gives the end of the book a sombre tone, an elegy of times and heroes gone and on the way to be forgotten - written in a prime example of Old English verse.
Maybe not meant to be experienced in this order, the four items certainly form a strong whole, one essential to the understanding of the author - more so than anything you might see in cinema these days...