Which "Merlin"?
There was "Merlin Ambrosius", the Ambrosius of Nennius who prophesied
for Vortigern, there was the "Merlin" who supposedly conspired actively
in the conception of Arthur (a crock of shit, historically speaking),
and "Myrddin Wyllt" or "Merlin Caledonius/Sylvester", who is actually a
composite of two individuals himself, one being the bard/sorceror of
Gwendolleu ap Ceidio, king of Selgovia (the tribal kingdom of the
Selgovae, north of Hadrian's Wall on the west side), and the other
being a character named "Lailoken", who is otherwise associated with
Strathclyde and Rhydderch Hael.
Strictly speaking, as a "sorceror", MERLYN would have been trained as a
bard in whatever remnants remained of the ancient druidic traditions,
and would probably have been a priest of the ancient pagan British gods
, and practice a form of shamanism (the "Myrddin Caledonius" associated
with Gwendolleu appears to have been a shaman/priest, rather than a
pure bard/druid type -- surviving poems attributed to him mention a
companion pig and wolves; the individual known as "Lailoken" in
Strathclyde apparently was linked to stags, and the name MERLYN
translates as "pony", which links to the worship of the Celtic sea-god
Manannan, who was associated with horses, swans, herons, and goats).
Druids did not write _ANYTHING_ down, and were primarily priests, who
indulged in a bit of cursing/curing during the course of their life as
priest/bard/healer/royal couselor.
"Spellbooks" _per se_ belong to the later Christian period,
particularly the late medieval/early Renaissance period, when the great
Rosicrucian/witchcraft craze was in full swing. In my research into
the witchcraft persecutions, I don't remember Robbins citing a single
case in the early part of the period in which there was an
authenticated "spellbook" of a witch. There are authentic GRIMOIRES of
the later period which are generally attributed to ceremonial magicians
who practiced various magickal traditions, such as alchemy, "Satanism"
(which is basically a perverted Christianity), "goetia", kabbalism, and
necromancy. Divination or prophecy does not require a spellbook --
sorcery or ceremonial magick of a complex type does. Neither does
your basic curse -- it's when you get involved with Judaeo-Chirstian
demonology and complicated magickal associations that "bleed over" from
ceremonial magick into witchcraft that you need a "spellbook".
And spellbooks imply LITERACY. Just how much of the British populace
do you suppose was literate in the 5th/6th century? In what language?
What script?
I think you really ought to do a bit more reading in REAL HISTORY of
the period, son.