Overhaul Changes

Generally the spellbook adds some really useful utility and Hide in Plain Sight at level 8 makes it useful to advance this class further.

Spellbook

From the source it is:

Spells list from the Steam project is pretty good, possibly too good even:

Assassin Spell List
1st: Ghostly Visage, Flare, Camouflage, Expeditious Retreat
2nd: Fox's Cunning, Cat's Grace, Darkness, Invisibility
3rd: Displacement, Blade Thirst, Poison, Hold Person
4th: Freedom of Movement, Improved Invisibility, Mass Camouflage, Haste

Source:

Spells: Beginning at 1st level, an assassin gains the ability to cast a small number of arcane spells. To cast a spell, the assassin must have an Intelligence score of at least 10 + the spell's level, so an assassin with an Intelligence of 10 or lower cannot cast these spells. Assassin bonus spells are based on Intelligence, and saving throws against these spells have a DC of 10 + spell level + the assassin's Intelligence modifier (if any). When the assassin gets 0 spells of agiven level, such as 0 1st-level spells at 1st level, the assassin gets only bonus spells. An assassin without a bonus spell for that level
cannot yet cast a spell of that level. The assassin's spell list appears below. An assassin prepares and casts spells just as a wizard does.

Assassin Spell List
Assassins choose their spells from the following list:
1st level—change self, detect poison, ghost sound, obscuring mist, spider climb.
2nd level—alter self, darkness, pass without trace, undetectable alignment.
3rd level—deeper darkness, invisibility, misdirection, nondetection.
4th level—dimension door, freedom of movement, improved invisibility, poison

Overhaul Description

(PRESTIGE CLASS)
The assassin is a master of dealing quick, lethal blows. Assassins often function as spies, informants, killers for hire, or agents of vengeance. Their skill in a variety of dark arts allows them to carry out missions of death with shocking, terrifying precision. Rogues, monks and bards are ideal candidates to become assassins.
 
WARNING: To cast a spell, a assassin must have a Intelligence score of 10 + the spell's level. For example, to cast a 2nd-level spell, a assassin must have a Intelligence of 12.

- Hit Die: d6.
- Proficiencies: Simple Weapons, Light Armor Proficiency.
- Skill Points: 4 + Int Modifier.
- Class Skills: Appraise, Bluff, Craft Armor, Craft Trap, Craft Weapon, Disable Trap, Heal, Hide, Intimidate, Listen, Lore, Move Silently, Open Lock, Pick Pocket, Search, Set Trap, Spot, Tumble, Use Magic Device.
- Primary saving throw(s): Reflex.
- Base attack bonus: Medium (+3/4 per level).
- Spellcasting: Assassin Spells. Arcane (Intelligence-based, spell preparation is required, spell failure from armor is a factor); assassins begin knowing 3 plus the assassins intelligence modifier 1st-level spells. They also learn 2 spells per level and can learn spells from scrolls.

REQUIREMENTS:

Alignment: Any evil.
Skills: Hide 8 ranks, Move Silently 8 ranks.

ABILITIES:

Level
1: Death Attack +1d6 - Special sneak attack that has a chance of paralyzing opponent.
  Use Poison - Automatic success when coating a weapon with poison.
2: Uncanny Dodge 1 - Retain dexterity bonus to armor class, even if flat-footed.
  +1 Fortitude save vs. poison.
3: Death Attack +2d6 - Death attack improves.
4: +2 Fortitude save vs. poison.
5: Death Attack +3d6 - Death attack improves.
  Uncanny Dodge 2 - +1 reflex saving throws, plus you resist sneak attacks from lower level attackers.
6: +3 Fortitude save vs. poison.
7: Death Attack +4d6 - Death attack improves.
8: +4 Fortitude save vs. poison.
 Hide in Plain Sight
9: Death Attack +5d6 - Death attack improves.
10: Uncanny Dodge 3 - +2 reflex saving throws.
  +5 Fortitude save vs. poison.

EPIC LEVELS:

You are the epic assassin, capable of sliding from shadow to shadow; lying in wait until your target is vulnerable, then striking like a cobra, leaving only a cold corpse behind.

Hit Die: d6.
Skill Points at Each Additional Level: 4 + Int Modifier.
Bonus Feats: The epic assassin gains a bonus feat every four levels after 10th.
Special:
 Sneak Attack: improves by +1d6 every two levels after 9th.
 Poison Saving Throws: an additional +1 for every two levels after 10th on the assassin's saving throws against poison.

Epic Assassin Bonus Feats List:
Epic Reflexes, Epic Skill Focus (hide), Epic Skill Focus (move silently), Great Dexterity, Improved Combat Casting, Improved Sneak Attack, Self-Concealment, Superior Initiative.