If your book is on a niche topic, check out the Oxbridge Directory of Newsletters, a resource book at your local library. You can search print newsletters by topic. Send copies of your book for review to newsletters, which have a loyal following.
Visit https://www.mediafinder.com/secure/product1.cfm#newsletters

If you've written a book targeted to senior citizens, visit http://www.seniornet.org. This is a book club for seniors that features discussion groups and even invites authors to discuss their books.

Don't forget all those college newspapers if you have a book targeted to college students. Collegiate Presswire provides press release distribution to college newspapers. Visit http://www.cpwire.com. The Associated Collegiate Press lists lots of newspapers. Click on "Members Online" at http://www.studentpress.org/acp/ The Editor & Publisher International Yearbook, available for purchase in print or on CD, includes a section for college newspapers. This publication also lists daily and weekly newspapers in the U.S., Canada and many foreign countries. http://www.mediainfo.com/store/store.htm

For more ways to target your niche audience, see special report #40.