What is Hip by Tower of Power. Actually most of Tower of Power is great. Roccio Prestia is considered one of the best fingerstyle funk players.

Also check out early James Brown. Cold Sweat P1, Licking Stick, Get up offa that thing...
What Is Hip is a beast of a tune but will push you so far so fast - it's a challenge at first, but I found my playing improve dramatically after a few weeks working on it.

Some other tunes...

Earth, Wind and Fire - Help Somebody
Jamiroquai - Whatever It Is, I Just Can't Stop
Jamiroquai - Too Young To Die
Stevie Wonder - Sir Duke

I also tend to recommend listening to this great Australian funk group named The Bamboos. Although I'm a little tired of their music at this point they have a seriously funky formula and you can learn their tunes by ear very easily - a workout you should never neglect.

I also suggest listening to soul jazz, particularly groups that incorporate a Hammond B-3. Those players bring the funk groove like no other.

But there's only so much you can do on your own - get out and play with some folks! A serious drummer will bring your playing along like nothing else.
That Anthony Vitti book is a beast. Be ready to scream in despair several times working through it. But it will make you a much better fingerstyle player when you finish.
Nice one... thanks guys.. Sir Duke by Stevie Wonder is a Beast of a song.. in fact i've found alot of his songs are serious fun to play.
The chorus of Sir Duke gets pretty hairy, and that unison line can be downright infuriating. Just sit down and play it over and over, slowly at first and building up to speed. It's a great tune to have under your belt - my funk band was rehearsing a few weeks ago and the singer asked if we would like to cover Sir Duke. The horn players knew a bit of the unison lick but nobody knew the whole tune... except me. Little things like that go a long way in a group.

Another thought I've had is that although we (bass players) want to jump right out and bring the funk it's just as important to swing. A good sense of time and swing are the foundation to lay down some serious funk. With that in mind, and going back to my recommendation of soul jazz, check out these records -

Jimmy Smith - Back To The Chicken Shack
Crash and Dr. Lonnie Smith - The Doctor Is In
Galactic - Coolin' Off
The Budos Band - The Budos Band & The Budos Band II
Big Sam's Funky Nation - Birth of a Nation
The Brother's Johnson - Blam !!
Michael Jackson - Off The Wall
Lotus - Nomad

Fairly disparate, they move across jazz, funk, disco, rock, afrobeat and a ton of other styles, but they all demonstrate excellent groove - these players are so funky that windows shatter and ghouls rise from the grave to tear it up on the dance floor.

We often forget that our record players are the most useful tool available to a musician - listening is just as important as playing. Whatever you're listening to, put down your bass and work through the music. Listen to the connection between the bass and drums, hear how the musicians interact with each other and collectively produce beautiful sounds. Then pick your instrument back up and start working out how those sounds were made, how you can use them to express yourself.