Three of Aguado's four methods are online. The Escuela de Guitarra is available in three editions, the 1825 Madrid edition in Spanish and the Spanish and French versions of the 2nd edition published in Paris the following year. You want the 2nd edition in either language; there are far fewer printing errors in them than the Madrid edition. The Escuela distinguishes itself from Aguado's other methods with its exhaustive and detailed beginner's study. There isn't a single 19th century method I would throw at a complete beginner... but this one comes the closest. There is a modem edition containing music only from Editions Orphee.
His 2nd method, the Nuevo Methodo de Guitarra Op.6 is not available anywhere online that I know of. There are plenty of links for Aguado Op.6, but they all lead you to the 1843 Nuevo Methodo PARRA Guitarra, which is a completely different book. That includes the link here at Delcamp. The liner notes and track listing of Lorenzo Micheli's Aguado CD repeat this annoying bibliographical error, which no one seems in a hurry to correct.
The method itself is a small, simple book intended for the unambitious amateur. It seems to assume the student already plays some guitar. It's mostly of historical interest, but it has some nice student pieces unavailable elsewhere. To the best of my knowledge it is not available anywhere online, and it can only be had in Chanterelle's Dionisio Aguado Complete Works Volume 1. It was hard to find for a long time, and I only recently got a copy myself (actually my lady picked it up but it amounts to the same thing). The method was published around 1834, and a bunch of opuses full of simple music were published simultaneously to go with it.
The La Guitare Ensiegnee par une Methode Simple came out just a few years after Op.6 and is cut from much the same cloth; a simple method to help the amateur play more interesting music. Why Aguado wrote two such similar methods so close together is kind of beyond me, but this is arguably a better book than Op.6 and is available online in several languages including an edition in English.
The Nuevo Methodo Parra Guitarra (not to be confused with the Nuevo Methodo DE Guitarra Op.6 except everyone does dammit) is his best known work and is famous for the exhaustive, almost anal retentive detail in which Aguado covers every possible aspect of guitar technique and for how close his approach is to the way guitar is played today. I think it's still the best intermediate to advanced manual for the self-taught student; I worked all the way through it after finishing with Solo Guitar Playing myself. It's available online in a number of editions, some of which stray pretty far from the original so looking for an early edition is advised (although really I'd say to hell with the facsimiles and get the modern Tecla edition in English). It was published in 1843, with appendix by Aguado added posthumously in 1850 so look for the appendix either loose or in an 1850 edition.