Know any books about Time?

Do you know any books that have a theme of time and time travel, possibly in a somewhat sci-fi theme (no romantic stuff like "time traveler's wife") it's fine if they are YA books.

I just read "the doomsday book", and I'll soon find the other book by the same author, but between that and Doctor Who, I think I'm hooked to the "genre"

I loved " A Tale of Time City" by Diana Wynne Jones, and "the Time Machine" (everyone knows who by).

Another kind I like is where people make contact across time, like when a girl moves into a home, finds some old letters of a previous owner, or seeing a temporal ghost, and later starts going through doorways and finding herself in the past, but not wholly there... usually with a broken clock involved that beats a 13th hour, or that suddently starts ticking backwards. (These are more like "ghost stories")