When is a book a tree?
The obvious answer to ‘when is a book a tree?’ is ‘before it’s been made into a book’ – it doesn’t take a scientist to know that (most) paper comes from trees – but things get more complex when we turn our attention to etymology.
The word book itself has changed very little over the centuries. In Old English it had the form bōc, and it is of Germanic origin, related to for example Dutch boek, German Buch, or Gothic bōka. The meaning has remained fairly steady too: in Old English a bōc was a volume consisting of a series of written and/or illustrated pages bound together for ease of reading, or the text that was written in such a volume, or a blank notebook, or sometimes another sort of written document, such as a charter.
The argument for…
The pages of books in Anglo-Saxon times were made out of parchment (i.e. animal skin), not paper. But nonetheless a long-standing and still widely accepted etymology assumes that the Germanic base of book is related ultimately to the name of the beech tree. Explanations of the semantic connection have varied considerably. At one point, scholars generally focused on the practice of scratching runes (the early Germanic writing system) onto strips of wood, but more recent accounts have placed emphasis instead on the use of wooden writing tablets.
Words in other languages have followed this semantic development from ‘material for writing on’ to ‘writing, book’. One example is classical Latin liber meaning ‘book’ (which is the root of library). This is believed to have originally been a use of liber meaning ‘bark’, the bark of trees having, according to Roman tradition, been used in early times as a writing material. Compare also Sanskrit bhūrjá– (as masculine noun) ‘birch tree’, and (as feminine noun) ‘birch bark used for writing’.
This explanation has troubled some scholars. There are two main reasons for this. Firstly, the words for ‘book’ and ‘beech’ in the earliest recorded stages of various Germanic languages belong to different stem classes (which determine how they form their endings for grammatical case and number), and the word for ‘book’ shows a stem class that is often assumed to be more archaic than that shown by the word for ‘beech’.
Secondly, in Gothic (the language of the ancient Goths, preserved in important early manuscripts) bōka in the singular (usually) means ‘letter (of the alphabet)’. In the plural, Gothic bōkōs does also mean ‘(legal) document, book’, but some have argued that this reflects a later development, modelled on ancient Greek γράμμα (gramma) ‘letter, written mark’, also in the plural γράμματα (grammata) ‘letters, literature’ (this word ultimately gives modern English grammar), and also on classical Latin littera ‘letter of the alphabet, short piece of writing’, also in the plural litterae ‘document, text, book’ (this word ultimately gives modern English literature).
In light of these factors, some have suggested that book and its Germanic relatives may show a different origin, from the same Indo-European base as Sanskrit bhāga- ‘portion, lot, possession’ and Avestan baga ‘portion, lot, luck’. The hypothesis is that a word of this origin came to be used in Germanic for a piece of wood with runes (or a single rune) inscribed on it, used to cast lots (a practice described by the ancient historian Tacitus), then for the runic characters themselves, and hence for Greek and Latin letters, and eventually for texts and books containing these.
However, many scholars remain convinced that book and beech are ultimately related, and argue that the forms and meanings shown in the earliest written documents in the various Germanic languages already reflect the results of a long process of development in word form and meaning, which has obscured the original relationship between the word book and the name of the tree. For some more detail on this, and for references to some of the main discussions of the etymology of book, see the etymology section of the entry for book in OED Online.