Jeff,
Gosh where to start...
Maybe you'll indulge a little history. There are basically 3 streams of classic soul music (vastly over-simplified, there are all kinds of other regional soul hot-beds):
1. The Detroit sound (i.e. Motown). Soul music polished up and made pretty mainly for a white teen audience, owned by black owner (Berry Gordy) and music made by black performers/writers. Arguably a bit vanilla, but only when compared to the "deep" Southern soul sound. Still, backed by the Funk Brothers, and with all the classic songwriters they had, this is still some of the best music made. Their stars were sent to finishing school, and were always respectable role models dressed well. Berry Gordy didn't want to alienate the white audience or white radio. You've got the Temptations, Supremes, Smokey Robinson (whom Dylan called the greatest pop-music writer ever), but especially Marvin Gaye and Stevie Wonder, who in the early 70's going forward, won their artistic freedom and took the music to a whole new level. The Marvin Gaye box set, The Master is fantastic. The Stevie Wonder Box Set, At The Close Of A Century is also good, but his work takes a quality dip in the early 80's making the last CD kind of patchy. But discs 2 and 3 are mind-blowing, he's not called a "genius" for nothing.
2. Sothern Soul. Black music for the black market, made by integrated groups, often called "gut-bucket" due to its gospel and blues foundation. You have the Memphis sound, dominated by the mighty Stax label, and the Muscle Shoals, Alabama sound, the deepest soul of all, centered around Fame Studios. Stax had one of the best house bands, Booket T. and the MG's, they played on and wrote most of what came out of 60's Stax. White guitar and bass player with a black keyboard player and drummer. You've got the best of all, Otis Redding out of Stax, along with Sam & Dave, Wilson Pickett, and a million more. The Complete Stax/Volt Singles 1959-1968 is the Rosetta Stone, but it's 9 CDs and it ain't cheap. They don't cherry-pick, you get everything that came out of Stax, you can chop it down yourself to a great 5 or 6CD set.
The Muscle Shoals musicians were also integrated. Aretha Franklin went down south after years of unsuccessful playing of uptown Jazz to switch to dirty, gritty soul, and she found her true voice and her legend started. Percy Sledge is also a great example of this style. I'd suggest the concise Very Best Of on Rhino from '98. The great Etta James did a lot of work at Fame, there's a CD exclusively of those sessions called Tell Mama: The Complete Muscle Shoals Sessions from 2001 that is red hot (but some filler). Her box set, The Chess Box is better, though, covers her whole career at Chess including the stuff she cut at Fame.
3. Philadelphia soul. Smooth, rhythmic more uptown, but funky, danceable horn-driven, eventually morphed into disco. Starts in the late 60's peaks in the early 70s (tends to be better-recorded, too). Dominated by the Thom Bell sound and the Gamble & Huff sound, with artists like the O'Jays, the Isley Brothers, the Spinners and the Stylistics. There is top-notch 3CD Gamble & Huff anthology called The Philly Sound 1966-76. The O'jays put out a million great early to mid 70's albums, all in remastered form, but I chielfy recommend Ship Ahoy from 2001. There is an SACD version that is amazing, if you have SACD. Same is true of the Isley Brothers best 3x3, the SACD is king, but the 2001 remaster not far behind. Very well-recoded albums, both.
OK, all that said, as first steps, I'd really consider the following:
1. Otis Redding Dreams to Remember 2CD set.
2. The Philly Sound box set.
3. Beg, Scream, Shout! 60's soul box set from Rhino, a kind of Holy Grail, very rare and expesnive but 6CDs of the best. It's the easiest way to collect this stuff, but it focuses more on the obscure artists, so you'll also need:
4. Soul Spectacular, 4CD box set on Rhino from 2001. About 1 CD worth of overlap with Beg Scream Shout, these are the best-known soul classics.
5. Can You Dig It: The 70s Soul Experience from 2001. Soul music didn't stop in 1969, the early 70s were an equally fertile period, this is also an essential purchase.
These box sets will give you a flavor for the major artists in the genre, and you can go from there to explore their individual catalogs. When you see things on the Rhino label, that is generally a mark of quality, both sonically and especially in terms of presentation, their booklets are great. They specialize in a lot of this material. Scan ebay for the best deals.
*phew* Good luck, it'll be a fun journey filled with a million discoveries.
EDIT: P.S. For your reading pleasure, the definitive tome is Peter Guralnick's excellent Sweet Soul Music: Rhythm and Blues and the Southern Dream of Freedom. He's an excellent music writer, and soul music devotee, and brings this material to life.
