Barb Purdom's "Psychic Serpent" trilogy are fine, fine alternate Harry Potter fanfics. Full sized books, they replace books 5, 6 and 7 of the original series. There's no slash in them, though the characters
do act their ages as they get older (think, they're all hormonal teenagers!) so there is some fooling around (I wouldn't let younger readers at them).
Book 5 is
Harry Potter and the Psychic Serpent
Book 6 is
Harry Potter and the Time of Good Intentions
Book 7 is
Harry Potter and the Triangle Prophecy
In my opinion, these books are better than JKR's. Book 6 especially, is so different and amazing. If you like these, Barb also did a "prequel" novel that features the older characters (and Harry's parents) in their younger years called
The Lost Generation.
You can Google them and find them easily on the fanfic sites or in PDF format; I've also got them in zipped HTML files myself. They really are worth reading.
Edit - Stopped being lazy and did some googling myself:
http://www.fictionalley.org/authors/barb/
And the pdf's are here:
http://home.arcor.de/mark.halpaap/
2nd Edit - Found Barb's Yahoo groups, where she has many formats, including lit which should convert nicely in calibre to epub.
http://groups.yahoo.com/group/Psychic_Serpent/
http://groups.yahoo.com/group/Psychic_Serpent2/