For the first part:
The DD rules say
that scrolls can be "copied" thereby implying that they must be copied from a spellbook, or perhaps from an existing scroll. Nothing is said explicitly about the reverse process: copying a scroll spell into a spellbook. The ref could possibly assume the same time and costs are incurred as for creating a scroll; the main inconvenience being the amount "downtime" that the character must be spent out of game.
that magic-user, but not a cleric, must use a Read Magic spell to "unlock" a spell scroll before he can use it. This is apparent in any case because there is no clerical Read Magic spell).
And for the second part:
The most probable spellsbooks in game will contain spells of exactly one spell-level. This is partly because the treasure tables yield spellbooks by spell level (such as: "A book of 3rd level spells") and partly because the rules allow the player to construct new spellbooks by spell level.
A spellbook containing spells of mixed spells-levels would be possible only if the referee permits the player to write spells here and there, although--as mentioned above--this isn't explicitly allowed (or disallowed). On the otherhand, the rules do explicitly permit the player to construct brand new spells, but do not imply where
these new spells should be written. One approach is to assume that a new 2nd level spell is written into an existing book of 2nd level spells, but ultimatley this is up to the referee.
We might also notice that when high level magic-users are encountered they are alwasy accompanied by a number of apprentices. The rules state: "Bereft of his spell books he cannot memorize any spells!" so presumably these apprenctices are necessary in order to carry all of the Wizard's spellbooks about !
So, yes, I beleive it is reasonable to assume that a magic-user or cleric would possess
one spellbook for each spell level he has access to but this doesn't mean that he must
always carry them. He might choose to leave them at home or to have other people carry them for him
the Elf Medium (MV 12", AC 9, HD 1, hp 1/1, AL N) great cloak, lantern
; spells: color spray; scrolls: sleep, sleep, charm person
Hirelings: Georges; torch