In keeping with the clean and tight guidelines that is B/X D&D, I wanted to stay within the borders of the written rules, yet still explain my interpretation of a spell books size and weight to my players. Since I enjoy a bit of fanciful magic in a game, I figured why not have some fun with a spell casters most prized possession?
Here is the excerpt from my house rules concerning spell books:
"Due to the magical and costly nature of the materials needed to construct one of these tomes, a normal book is not suitable to store spells. Since most magic users do not possess great strength, spell books are magically crafted to have very little weight. For encumbrance purposes they are considered part of Misc Equipment (80 coins weight). Elven spell books also follow these rules. A spell book is bestowed on the fledgling caster by their mentor, after tutelage is completed, as they begin their adventuring careers.
In regards to the number of pages contained in a spell book, an interesting anomaly can be found - a spell book will always have just enough pages for a casters spells. During the ritual of inscribing a new spell, these books magically gain new pages as required. Thus, a spell book will contain spells equal to the number and level of spells a caster can use in a single day (pg X11)."
So that's it, short and sweet. I don't think this idea should be seen as a game breaker. Rather, it's just my way to explain the small details left out of the rules by the original authors and to inject a bit of whimsy into the magic user and elf characters. As always, thanks for reading!