[bjones6416]

From the Amazon Help pages:

The following is a partial list of merchants selling items at Amazon.com which may be included in your order, and the states in which they charge sales tax.

Amazon.com LLC: KS, KY, ND, NY and WA
Amazon Digital Services, Inc.: KY, ND, NY and WA (Kindle content, MP3s, and digital videos are only taxable in KY and WA)
Magazine Express, Inc.: AL and WA
Synapse Services, Inc.: WA only
Target.com: All states other than VT
Hachette Digital, Inc.: AL, AZ, CO, CT, DC, HI, ID, IN, KY, LA, ME, MS, NC, NE, NJ, NM, SD, TN, TX, UT, VT, WA, WI and WY*
Harper Collins Publishers, LLC: All states other than AK, AL, AZ, DE, HI, MT, NH, NV, OK, OR, SD, VT and WY*
Penguin Group (USA) Inc: All States*
Simon & Schuster Digital Sales, Inc.: All states other than AK, DE, MT, NH, and OR*
Macmillan: AZ, CO, CT, DC, HI, IN, KY, ME, MS, NC, NE, NJ, NM, SD, TN, TX, UT, VT, WA, WI and WY*
Zondervan Corporation LLC: CA, CO, FL, GA, IA, IL, IN, LA, MA, MD, MI, MO, NC, NV, OH, PA, SC, TX and WA*
* Kindle books sold by various publishers are subject to sales tax based on the publisher's state tax reporting obligations and the taxability of digital books in those states. As a result, sales tax for Kindle books sold by the publisher may differ from the sales tax to which you've been accustomed for Kindle books.

So, yeah, it looks like it depends on the publisher as far as Kindle books go...