Traditionally, couples gave each other gifts of paper on the first anniversary. This same tradition prescribes gifts of silver for the 25th anniversary and of gold for the 50th.
(See
http://marriage.about.com/od/anniver...nivhistory.htm for information on the traditions and a link to the list of gift traditions for all the other anniversaries.)
AT tokenz.com, they point out that the gift list escalates from inexpensive to more expensive as the number of years goes up. "The logic of presenting gifts was that stability deserves a reward and more the stability the greater should be the reward."
So -- a book would be a highly appropriate gift! I also agree with addicted2books that having a piece of jewelry (small or large -- doesn't matter) would be something tangible she could point to. One thought comes to mind: what about a charm bracelet? You could give her a bracelet with a single charm on it this year and dall it her "anniversary bracelet". Perhaps a charm of a book to go with an actual book. Then next year, add another charm. The charm could be based on the gift suggestions or maybe a memorable event that occurred during that year.
The tokenz.com site also lists jewel stones for each anniversary and you could work that list as well.
I'd also recommend a nice dinner out. The first couple of years, we went to the same restaurant on our anniversary -- one that we rarely, if ever, went to except on the anniversary. Sadly, the restaurant chain is no longer in business, and as we are approaching our 38th anniversary, we're a bit more casual except for the -5 and -0 ones. For #30, we went on a two-week tour of the Four Corners area, hitting the major National Parks. I think we need to start planning something memorable for #40.