I don't see anything at all wrong with changing positions while you read a book.
You stay limber and more alert that way... In other words, you won't have your hand or foot 'go to sleep,' you won't get tired before you're finished reading, and you really WILL stay more alert.
Keep on the way you have been, without asking yourself any questions about what position you should be in to read a book. (Some question [!])
Just be sure the light is sufficient, that's all.
EDIT ADD: I always and frequently change positions when I read, either for work or fun. (<--To answer the rest of your question)...