Recall that long ago, books were not printed on paper made from tree pulp or bound in bindings, but instead for thousands of years prior to the printing process and paper making, 'books' were hand copied on scrolls of skin or cloth (see 卷), having no significant connection to trees or wood other than the two woody shafts on each side connected by skin. This glyph formerly appears to have been drawn as「two-handed」holding something with both arms stretched in a「big」shape, with「twisting, screwing, filled」(夲), both of the hands scrolling two wooden shafts attached to each end of vellum or parchment (old-style BOOKS as two wooden shafts connected by skin).
Shaman accounting systems kept the BOOKS used to record client visits for time-and-billing purposes, not coincidentally applying to each「woody shaft.」In modern times, this glyph is still used as a counter for home runs, goals, scoring, and long cylindrical objects for this very reason. Entire trees are not generally cylindrical in shape, but rotating or screwing shafts normally are, and a tree trunk stripped of all its branches and roots may be. Somewhat unrelated to trees but not unrelated the PRIMARY method applied to one's MAIN STEM, SHAFT might be described as「two-handed screwing.」ORIGIN also applies to the main stem of a woody, the SOURCE of life essence, a male's BASIC, PRIMARY concern for progeny as well as recreation, and the elements today appear to be「woody screwing, rotation of shaft.」
Meaning of BOOK may also be derived from the SOURCE of knowledge, the ORIGIN, BASIC, PRIMARY source prior to the days of the Internet. LONG SHAFT OF WOOD, resulting in wood shafts (木) inserted into a center hole (中) deep within two skin vortices (巴, one on each side of the scroll). This is the reason Chinese and old Japanese was written vertically from right to left: rolling the scrolls allowed a fluid reading of all text content.
Perhaps a marker stroke or「wide」on the TRUNK of the「tree, plant, woody」glyph as the scholars insist, the trunk or main stem being the ORIGIN and PRIMARY structural part that connects the leaves and branches with the root system…except that the trunk is not in fact the origin of a tree, and it can successfully be argued that it is also not primary, since the elimination of either the branches and leaves, or the root system, will result in the death of the organism in most cases. Cut down a tree to a low stump, and more often than not the roots sprout new growth for years to come.
And frankly, it would not be beyond belief to point out that the ancient Chinese may have used this glyph as a part of the designation for Japan due to the contour of the country having an uncanny resemblance to a「woody」with the two island areas of Kyūshū and Shikoku as testicles, Honshū as the main shaft, Hokkaidō looking like the glans head, and the northern outer islands as drops spewing and spurting from the meatus. Many Japanese themselves liken the shape of their country to a dragon: see 竜 for details. Chinese liken a dragon to a schmekel, the shaft of a woody.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: roots or stems of plants; origin; source; this; the current; root; foundation; basis; classifier for books, periodicals, files etc; originally
Japanese EDRDG: book, present, main, origin, true, real, counter for long cylindrical things
Unihan extended: root, origin, source; basis

Used within names (nanori)
まと　･　ごう