This is Part Two of a critique of the Delving Deeper rules system and the changes from the original rules it includes. Part One was posted some time ago and covered character classes.
Delving Deeper naturally keeps the well-known Vancian magic system pretty much intact. It even makes explicit that clerics have spell books, something that's hinted at in the original rules, but dropped from later editions. The cost of creating a scroll is 50% higher than the original rules, and DD also applies the "cost per spell level" of AD&D, implying that each spell is on a separate page or multiple pages, as in AD&D. This is in contrast to the flat cost for an entire spell book for a given level, which is in the original rules. These are, of course, minor changes and fairly easy to change back, but there is another, more significant, change: spells can be cast by reading from a spell book, which erases the spell from the book as if it were a scroll. This is certainly not the way I'd run spell books, and I'm not even sure if I could recommend it to others as an emergency spell casting rule. Because of the change in spell costs, it turns a spell book into a collection of slightly over-priced scrolls, with no significant downside. If the minimum cost for a replacement 1st level spell book is 2,000 gp, or if you use the new costs and casting from a spell book has at least a chance of erasing the entire book, it might be more palatable.
The cleric lists includes a few surprise reversed spells not in the original: Obscure Magic, Obscure Alignment, Obscure Traps, Obscure Object, and Obscure Evil, instead of the more typical (late-edition) Detect Good. Unfortunately, Protection from Evil goes with the late-edition interpretation, making it feel more like a rudimentary second alignment access, instead of "evil" being defined as hostile intent. Animate Dead gets a reversed form, Enervate Dead, and Enervate Reptile has been added so that the reversed form (Animate Reptile) can be used to emulate Sticks to Snakes. I do kind of like that change. Also, Neutralize Poison seems to be weaker, and there is no Slow Poison, so poison effects become much more serious in DD than in the original.
The Sleep spell uses an interpretation I've seen floating around: The GM rolls for the minimum number of hit dice affected, and depending on positioning, the spell may affect the caster's allies as well. I haven't used that interpretation, but I'm tempted; it's a good balance for allowing Sleep to take effect without allowing a saving throw, although this is not hinted at in the DD version of the spell.
The Phantasm spell replaces Phantasmal Forces. In some ways, it's scaled back: no illusion of a squad of warriors, only one warrior at most. In other ways, it's a little tougher: DD resolves the issue of illusory damage by saying that disbelieving an illusion allows a saving throw, and if successful, the one disbelieving can dispel the illusion by touching it. By implication, failing the saving throw means the illusion can't be dispelled by touch, so it can also do damage.
The ESP replacement, Sixth Sense, specifies that the spell detects the contents of thoughts (telepathy,) not just the detection of a thinking creature. Lots of people use this interpretation, so they might not mind the consequences: Clairvoyance and Clairaudience no longer allow viewing/listening through a thinking creature, but work more like the original Wizard Eye. This means that the only benefit of the DD replacement (Witch Eye) is that it is mobile.
Haste and Slow use the later-edition changes of adding or subtracting extra attacks. Protection from Normal Missiles no longer protects against boulders (it didn't protect against ogres or giants throwing boulders, originally, but arguably it protected against catapults.)
Confusion and Contact (Other) Plane seem significantly improved, at least from my point of view, because there are fewer die rolls involved. Baleful Polymorph (the equivalent of Polymorph Other) has been moved to a higher spell level. Elementals are now affected specifically by Dispel Evil -- but because of the changes to the interpretation of "evil" in cleric spells, this implies that elementals are evil, which may or may not fit with an individual campaign.
It's also mildly interesting that the Reincarnation spell can bring you back as an elf -- but not as the single-class elf that DD makes possible. There are other changes as well, like Control Water, but these don't seem very significant.
In short, I have a problem with the interpretation of "evil" in some spells and the rewrite of Haste/Slow and ESP/Clairvoyance/Clairaudience, like the changes to Sleep, Confusion, and Contact Other Planes, and can probably bear the other changes, although I personally would move Baleful Polymorph back to the same level as Polymorph (Self).
I'll be looking at the GM rules in Part Three.