Okay, here is one of my favorite old, , looong Sentinel fics (over 400k.)

No Islands Any More, by Betsy-TacyKelly.

It's slash. It's rape-fic.

It's a wild, wild ride.

Well-written, occassionally overwrought, but never trite.

It's at the archive.

http://www.852prospect.org/archive/archive/drama3/noislands.html

Whoever these author/authors are, this is the only fic by them I've ever found under this name, and AFIK, none of the big names have ever owned up to this one.

But somebody who knows how to write wrote this. It's memorable, and even after all these years, has a special place in my heart.

Control Issues (http://lit-gal.livejournal.com/?skip=40&tag=control+issues) by lit_gal
NC17; Jim/Blair; AU, Rape/Non-con*, Explicit Violence, Adult Themes;
In the words of lit_gal herself:
The United States is fair to its Sentinels, but given the history of abuse they've suffered, the state has a right to take some steps to protect them... even if that curtails a few rights. At least, that's the belief most people have. A chance encounter between a Runner named Jim Ellison who refuses to accept the party line and a hippy in an airport might just change the world.

This is an absolutely awesome story. It's well thought-out, realistic, and beautifully written. lit_gal deals tastefully and realistically with a common fannon cliche: Either Guides or Sentinels are enslaved or have fewer/no rights under the law. Most stories using this cliche are sensationalist, overly dramatic, and essentially implausible. Not so with Control Issues. lit_gal has done a wonderful job of making this cliche so realistic that the Readers can imagine that such situations could take place in their Real Lives. This is a thought-provoking look at things like privacy, the government, institutions, beauracracy, and inequality in our daily lives. Of course, the story is so wonderful that you probably won't even notice the fact that you're being prompted to think about these weighty issues until after the fact!

Definitely a must read - especially for Sentinel fans!
Just scroll down to the bottom of the tags page that the story link leads to in order to find the first chapter.

I think my all-time favorite Sentinel series (well, after cesperanza's Nature Series, but everyone's read that already!) is lanning's "True Minds" trilogy -- True Minds, Little Company, and Many Waters. All three parts are over 200k; there's angst and love and mysticism and happiness and lots of plot and really good writing.

Hee -- great minds think alike, huh? :-)

As an alternate, have you read The Aegis Group, by veronicaluv? It's an AU where Jim works for an elite research group that studies and tries to help Sentinels, among other things. He meets Blair when Eli Stoddard, who Blair has been working with, is murdered in Peru; Jim is dispatched to rescue Blair and get him out of the country. Anyway, it's a huge long epic, almost 500k, and really well-written.

I absolutely adore Habour of my Heart (http://www.852prospect.org/archive/archive/4/harborof.html) and Haven of my soul (http://www.852prospect.org/archive/archive/27/havenof.html) by romslinger. They are not connected but the overall theme is a bit similiar. Both are AU's, slash and HUGE.

In the first one Jim, an ex-army ranger, is living on the streets with his two adopted, adorable nieces. Detective Blair Sandburg takes them in.

In the second fic Jim meets 'The Professor' in his soup kitchen. Blair lost his memories so everyone calls him Professor.

Read them if you haven't yet, they're are so good.