In the original AD&D PHB & DMG, the only means described of doing so is the spell "Write" (which requires 200 gp).
Unearthed Arcana finally laid down rules for the spellbook: to transcribe a spell requires 100 gp per spell level. The Standard Spellbook (1000 gp before spells are added) takes 4-7 weeks to create and can contain 24 spells of level 1-3, 16 spells of level 4-6, or 8 spells of levels 7-9. (Encumbrance 450 gp)
The Travelling Spellbook costs 500 gp for the "blank" book, and takes the same time and cost as a Standard Spellbook to prepare. It can carry 6 level 1-3 spells, four level 4-6 spells, or two level 7-9 spells. (Encumbrance 60 gp - five of such will fit in a backpack).
Cheers!