Early on in my learning I used a nifty little book entitled "Basic Timing for the Pianist." It is a collection of "105 short, short exercises leading to thorough and complete mastery of basic timing problems." The author is Allan Small.
I enjoyed working with it and while I cannot say I mastered timing thoroughly and completely (as promised in the title), the exercises certainly did help!!!
I once had a rock oriented teacher that suggested using a drummers book for timing. He said it was useful for a well rounded musician that wants to improve timing skills. I did that for awhile and it helped.
I also used the book by Allan Small and preferred it. In my opinion, its a must have
for any keyboard player that wants to work on timing.