Lately I have been quite interested in the blades. I was wondering if there is any information in any of the games besides what the blades themselves have to say (and the occasional comments by other NPC), such as books? I haven't found anything myself, but maybe someone knows something?

Spies don't generally give information about themselves to civilians. So, I think pretty much all we know comes from the Blades themselves.

Your best source is playing Morrowind, and asking people about "Blades."

Here's http://imperial-library.info/category/tags/blades. The list is probably incomplete, but it's a start.

Yer basic skinny on the Blades is that they were formed by Tiber Septim, out the last remnant of Reman's Akaviri Dragonguard (namely the Chevalier Renald), as an order of grailseeker-types to round up the pieces of Numidium after the Underking broke Talos' toy. Then, one has to assume over a long period of time, their raison d'être shifted into espionage, bodyguarding, and (one also has to assume) assassination.

Thanks for the answers everyone, I will be sure to check that link out lady nerevar

The blades are the good guys. Whatever secretive Imperial order makes the most sense to be the one helping out the PC, that's the blades. I'm guessing we've a lot more to see from them yet. Weirdly, we've probably only scratched the surface.

How much more can they really do though? They started out as spies, then became bodyguards and now they are also dragonkillers... What's next? Food tasters, Bed warmers? Teachers? Priests? Hunters to provide food in the first place?

I'm thinking the blades will be a guerrilla army fighting to overthrow a victorious Aldmeri Dominion. A name for the entire resistance to unite behind. It fits with the general expansion of the scope of the Blades in each game.

There wouldn't be a free mind, in the entirety of Tamriel, to resist a victorious Dominion.

If ya wanna be technical one just so happened to be a priest as well as a Blade.

I'm hoping the Blades are done for. While i'd like to join a spy faction in a future TES game, I'd rather have the Penitus Oculatus. For one, they do seem more spy-ish (I can see the PO assassinating any potential threat, acting a bit like the Morag Tong - I see the Blades having more morals). Another reason is that the Blades wouldn't stand for the Aldmeri Dominion - I can the PO doing many contracts for the Thalmor, giving a more varied faction quest. Also, while they do spy as we see in Morrowind, they seem much more at home in an honourable battle with their armour and a katana, and the PO doesn't have that (from what we know so far anyway). And lastly, I think the blades should have died with the Septim Empire.

Isn't the PO Empire-aligned? I mean, just because Mede died doesn't mean the Medean Empire will die with him. He might have heirs, relatives, etc, who'll take the throne.

Yes, it's the Empire's "secret service" if you will. But i've got the idea in my head that the Thalmor constantly pull strings in the Empire behind the scenes, and that they have high ranking members in the PO, or the Empire allows them to work for the Thalmor. And I was refering to Uriel/Matrin Septim's death, and how the blades were pretty much destroyed with them and the PO rose from the ashes.

And one was a Skooma addict (still the coolest blade ever)... But can you just imagine the blades walking through the streets handing out leaflets to spread the gospel of Martin?

Not if Delphine and Esbern are in control of rebuilding the Blades. There's not much in the way of morals with the way they need and accept help from Paarthurnax, then immediately afterward turn around and demand that you (their leader) kill him or else they'll cut all ties with you (without whom they have no purpose), and calling it "justice" for something that happened thousands of years before their predecessors set foot on Tamriel and which affected the great-great-great-great-great-great-great-great-great-great-great-[...]-grandparents of some of the local populace and for which he's attempted to atone by helping save the world twice while remaining peaceful.

As of Skyrim, the Blades have no morals.

Once upon a time, no, I could not. But now that the Blades are in the *ahem* capable hands of Tweedledee and Tweedledum, I'm not so sure. "Excuse me sir, if we can have but a moment of your time, we'd like to explain to you why Dragons are a negative impact on our local community..."

This made me laugh.

Yup. The Blades have all but completely fallen. Delphine may be an effective warrior according to her dossier, but she's got such nasty PTSD from the War that she's useless in a command position, unstable and angry. Esbern, also, is more at home in the library of an organization run by someone else than as a leader, even if he is a nice grandfatherly type on the surface. They were so desperate for a purpose that they'd kill Paarthurnax based on an oath that had been defunct for at least 600 years despite all he had done."Lawful Stupid," to use a simple term. They need a shot in the arm or a desperate hope that one of their actual leaders managed to survive the Great War in another province and maintain his or her sanity to get them back on track. Otherwise, lament for greatness sullied and lost.

On the other hand, Delphine can get to artifacts before the player can, and magically knows what you just did before you tell her. And she and Esbern are both immortal, by means of somehow altering the notations in the Elder Scrolls to delete their own possibility of death, as far as I can figure. So... they aren't total yutzes. Just incredibly competent jerks pretending to be yutzes.

That's why the Thalmor already exterminated them.

I feel like the Penitus Oculatus (spelling error I'm sure) is going to be corrupt, and an emporer will use them as somewhat of a secret police. I don't know, I've just been feeling this sense of distrust with a lot of Skyrim characters recently

Congrats, no spelling error with the PO. But I get what you're saying. Might be that Titus Mede II's heir is no good.