Wizards learn about spellbooks as they enhance their spell skills

Previously, Wizards got a boost to the chance of writing unknown
spellbooks based purely on being a Wizard (with the chance still
luck-based), leading to a very large power spike when the Wizard
gained access to a luckstone and the ability to max out luck.
This had two main issues: this power spike came *after* the major
early-game difficulty spike, often leaving Wizards forced to deal
with it without having appropriate spells; and it promotes
grinding (for Luck and for Magicbane) at an early point in the
game, meaning that the Wizard early game effectively followed a
sequence of extreme difficulty -> grinding -> minimal difficulty,
which isn't very good balance-wise.

With this commit, Wizards lose their advantage to writing unknown
spellbooks by guessing, and instead learn spellbook IDs based on
their spell skills (advancing a skill gives knowledge of higher-
level spellbooks). This means that writing unknown spellbooks
becomes guaranteed with sufficient skill, but has no advantage
over non-Wizards in schools where the Wixard does not have
sufficient skill.

Due to Wizards' skill caps, there are two spells which they can't
ever write guaranteed: create familiar and charm monster. Create
familiar is a fairly niche spell (that doesn't match the Wizard
playstyle that well) and being unable to write it is not a major
problem. The inability to easily write charm monster is
intentional.

src/write.c

@@ -313,8 +313,8 @@ dowrite(struct obj *pen)
     /*
      * Writing by name requires that the hero knows the scroll or
      * book type.  One has previously been read (and its effect
-     * was evident) or been ID'd via scroll/spell/throne and it
-     * will be on the discoveries list.
+     * was evident) or been ID'd via scroll/spell/throne (or skill
+     * for Wizards) and it will be on the discoveries list.
      * Unknown spellbooks can also be written by name if the hero
      * has fresh knowledge of the spell, or if the spell is almost
      * forgotten and the hero is Lucky (with a greater chance than
@@ -349,7 +349,8 @@ dowrite(struct obj *pen)
         /* else fresh knowledge of the spell works */
         && spell_knowledge != spe_Fresh
         /* and Luck might override after previous checks have failed */
-        && rnl((Role_if(PM_WIZARD) || spell_knowledge == spe_GoingStale)
+        && rnl(((Role_if(PM_WIZARD) && paper->oclass != SPBOOK_CLASS) ||
+                spell_knowledge == spe_GoingStale)
                ? 5 : 15)) {
         You("%s to write that.", by_descr ? "fail" : "don't know how");
         /* scrolls disappear, spellbooks don't */