The details and subplots contained in the books (versus the streamlined movies) are helpful in rounding out what sometimes seemed like overly deus ex machina endings. This is particularly true of Dumbledore, who is just such a good wizard, it's a wonder why everyone's so concerned with Harry. The wrap-up after the finale in the Chamber of Secrets is both positive (for most characters) and rewarding because the book takes the time to develop its characters.
I enjoy populating my imagination with the faces and design of the movies, but I do wish I had read the books first. I trust I'll feel the same way if I ever get around to the Lord of the Rings trilogy. This situation reminds me of how when it comes to oft-covered songs, it's usually that which the listener hears first that they forever associate as "best". (This also applies to James Bond actors). The one exception to this has been the song "Blinded by the Light" which I first heard in the trippy, synthy Manfred Mann version. I eventually over time and many listens grew to love the Springsteen original better, despite not hearing it until high school. Maybe if I reread the HP series often enough, things will stop looking like the movies.
One more anecdote on this subject: One of my favorite books is Tom Sawyer, and, two summers ago, my imagination was in overdrive because of traveling, being in Greece, and otherwise being pretty unconnected from the numbing effect of TV and computers. Rereading the book then was really amazing, and I vividly saw the events of the novel play out in a sort of animated watercolor painting that was informed by the illustrations in a Reader's Digest edition I'd read as a wee lad, but had evolved into a unique style that only I experienced while reading. Power of the printed word.
