He is featured in the 1st, 2nd, 4th, 5th, 6th, and 7th book.
(Sorcerers Stone, Chamber of Secrets, Goblet of Fire, Order of the Phoenix, Half Blood Prince, and Deathly Hallows.) Harry doesn't actually meet him in The Prisoner of Azkaban or The Half Blood Prince.
Ralph Fiennes.
No, Lord Voldemort has only been played by Ralph Fiennes in Harry Potter films 4, 5, 6, and 7.
Harry Potter 4: Cedric Diggory. Harry Potter 5: Sirius Black. Harry Potter 6: Albus Dumbledore. Harry Potter 7: Lots of people; too numerous to list here - including Dobby the house elf; Fred Weasley; Nymphadora Tonks; Remus Lupin; and of course Lord Voldemort.
Ralph Fiennes.
(1) Lord Voldemort was a bit hard to find, and (2) as explained in later books, he had taken certain precautions that made it difficult to kill him (the horcruxes, explained in book 6).
1. Tom Riddle's Diary 2. Marvolo Gaunt's Ring 3. Slytherin's Locket 4. Helga Hufflepuff's Cup 5. The Diadem of Rowena Ravenclaw 6. Harry Potter himself 7. Nagini the snake
Ralph Fiennes plays Voldemort, Tom Felton plays Draco Malfoy, Jason Isaacs plays Lucius Malfoy, Helena Bonham Carter plays Bellatrix Lestrange, Joshua Herdman plays Gregory Goyle, Nick Moran plays Scabior, Peter Mullen plays Yaxley, Helen McCrory plays Narcissa Malfoy, Jamie Waylet plays Vincent Crabbe, ...
yes, the other horcruxes are:1. Tom Riddle's Diary 2. Marvolo Gaunt's Ring 3. Slytherin's Locket 4. Helga Hufflepuff's Cup 5. The Diadem of Rowena Ravenclaw 6. (unknown to Voldemort) Harry Potter himself. 7. Nagini the snake
Snape becomes head master and controls the school with two other death eaters Alecto and Amycus while Voldemort tracks down Harry Potter.
Lord Voldemort uses an old form of magic to return. Peter Pettigrew sacrifices his hand, they forcibly take blood from Harry Potter and use a bone from his fathers grave. This, and Voldemort's body are added together in a potion and Voldemort emerges.
Harry, Ron, and Hermione need to find the 6 (of what they know) Horcruxes and destroy them. they found and destroyed them all to defeat Voldemort.
In Harry Potter and the Half Blood Prince, Draco Malfoy, Voldemort of course, and until the 7th book, You find out that Snape was never a bad person and was only working undercover for Dumbledore and he had a deep love for Harry's mom. :P