[RPG] Are there any Handbooks for the “Expert” NPC Class

NPC Classes are used for NPCs that don't play a major role in the story

The idea behind NPC classes are that they are not as powerful as PCs, and that they should be used for minor characters in the story. For example, in Pathfinder, the Adept class will be what most "priests" in a given church would be. They have some minor powers, and can cure wounds, but if you really need healing you'll need to go find an actual Cleric, which may be really hard to find in a small town.

Likewise, most city guards won't have PC levels, they'll have NPC warrior levels.

Most "normal" people will be Level 1 commoners.

You would not use NPC classes for a major villain

Major movers and shakers will have PC levels. If you are making a powerful adversary, NPC classes are not the way to go. If you've got Pathfinder, have a look at the creating NPCs section, and you'll see near the back there is an example of how to give an NPC regular Class levels. (Note that they still tend to have lower stats unless you choose to give them better ones).

What are tiers?

Tiers are a ranking of how "powerful and/or versatile" the various 3.5 base classes are, with low numbered tiers being considered more capable than high numbered tiers. It's important to remember that certain caveats apply to the rankings:

• Tiers assume similar levels of optimization. Someone playing an optimized "weak" class (like a fighter) and using its abilities well may be a lot more effective than a poorly built wizard played by someone who doesn't know how to make use of its options.
• Tiers attempt to describe power over levels 1-20. Classes will generally be in their listed tiers immediately, though the gaps between tiers tend to be a bit smaller at lower levels.
• Tiers are based on published material only. Homebrew and house rules can and will modify the rankings of some classes or even just negate the entire ranking system.
• Tiers are based on relatively high-magic games. In a low-magic setting the rankings will be mostly the same, but the gaps between tiers will get a lot bigger, because magic items tend to be the best way for less powerful classes to cover up their weak spots.
• Tiers look at characters' ability to solve problems of any sort, not just combat.

We frown on link-only answers, so I'll go ahead and summarize the full tier list of all published classes, originally from here. Fuller descriptions of why each class is in its tier can be found here.

Tier 1: Wizard, Cleric, Druid, Archivist, Artificer, Erudite (Spell to Power variant) — Can do anything and everything, often better than lower-tier classes that supposedly specialize in that thing.

Tier 2: Sorcerer, Favored Soul, Psion, Binder (w/ online vestiges), Erudite — As powerful as tier 1, but no one build can do everything.

Tier 3: Beguiler, Dread Necromancer, Crusader, Bard, Swordsage, Binder, Ranger (Wildshape variant), Duskblade, Factotum, Warblade, Psychic Warrior, Incarnate, Totemist — Good at one thing & useful outside that, or moderately useful at most things.

Tier 4: Rogue, Barbarian, Warlock, Warmage, Scout, Ranger, Hexblade, Adept, Spellthief, Marshal, Fighter (Zhentarium variant) — Good at one thing but useless at everything else, or mediocre at many things.

Tier 5: Fighter, Monk, Ninja (both CA & Rokugan versions), Healer, Swashbuckler, Soulknife, Expert, OA Samurai, Paladin, Knight, CW Samurai (with Imperious Command), Soulborn — Good at one rarely applicable thing, or mediocre at one thing, or simply too unfocused.

Tier 6: CW Samurai, Aristocrat, Warrior, Commoner — Objectively worse at their specialty than another (often Tier 5) class, without anything else to show for it.

Tier 7: Truenamer — Apparently received no actual playtesting, mechanics as written simply don't work. See this question for more details.