Have you been to
http://silenthillchronicle.net/ ? Check "Inspirational Works" near the bottom, there's a huge list of books, films, music and more the original Konami teams drew from. You can read scans of the same book (minus a few of the later chapters)
here. Goes without saying, both links are rife with spoilers for SH1-4. Sorry if they're old news, but it's always worth mentioning on the off chance someone's not seen them!
As for me, I love the first four games - particularly the second and fourth, maybe a little less the third since its story is a bit thin (but the more overtly terrorising atmosphere makes up for that - some of the ambient noises creep me out no matter how often I've heard them). I've just finished replaying SH1-4, as I tend to every year or so, still noticing new things about each game. They're the perfect alternative to shooters / other more technically demanding games, I find.
I haven't gotten along with the post-movie games, so far. All three have had good/great ideas, but have consistently limited themselves by clinging to the movie and increasingly tired elements of the earlier games (many used in the movie). The series can't take the risks it once did, sadly. Sales started tanking around SH3, if I remember right.
I consider it a dear but currently MIA series, and to be honest, four quality games in a horror series is good enough for me. Will be interesting to see if the upcoming game finally carves out its own niche, ala SH2 and to a large extent SH4.