Implement #624: Allow browsing of identified "risky" spellbooks.

* If you encounter the book of Annihilations/Demonology or Necronomicon
  in a bookshop, you can now browse its contents.
* If you buy such a book, it is already identified, so you can continue
  to read the spell descriptions at your leisure.
* You can still not memorise these spells if you don't fulfil the
  requirements. They will also be coloured useless while that is the
  case.
* If you find these books lying around in the dungeon, reading them will
  _not_ identify them (or give away their spell list) unless you meet
  the requirements. [1]

I also added the old "beyond your current level of understanding" line
to both book and corresponding spell descriptions.

Tested for the Necronomicon seen in a shop, Demonology seen in a shop
and bought, and Annihilations found unidentified (in wizard mode),
using (r)ead, (i)nventory, (M)emorise, ?/, and the Tiles mouseclick
interface (again, read/describe/Memorise commands and Memorise tab).

[1] I don't know if it's worth keeping that restriction. If we do, we
should add an autoinscription because the simple act of (r)eading it
will give away that it's one of those three special books, though not
which one.

1 files changed, 7 insertions, 3 deletions

diff --git a/crawl-ref/source/tilereg.cc b/crawl-ref/source/tilereg.cc
index 4d4c946f5f..160c301603 100644
--- a/crawl-ref/source/tilereg.cc
+++ b/crawl-ref/source/tilereg.cc
@@ -3238,10 +3238,14 @@ bool InventoryRegion::update_tip_text(std::string& tip)
                 if (item_type_known(item)
                     && item.sub_type != BOOK_MANUAL
                     && item.sub_type != BOOK_DESTRUCTION
-                    && you.skills[SK_SPELLCASTING] > 0)
+                    && can_learn_spell(true))
                 {
-                    tip += "Memorise (%)";
-                    cmd.push_back(CMD_MEMORISE_SPELL);
+                    if (player_can_memorise_from_spellbook(item)
+                        || has_spells_to_memorise(true))
+                    {
+                        tip += "Memorise (%)";
+                        cmd.push_back(CMD_MEMORISE_SPELL);
+                    }
                     if (wielded)
                         _handle_wield_tip(tip, cmd, "\n[Ctrl-L-Click]", true);
                     break;