Leigh, you touch on my feelings on the subject, that thank you notes in their current form are a burden not a thank you. In my opinion, any gift that creates an obligation (either a thank you or a return gift) is not truly a gift but a contract. To be clear, I've written many a thank you in many different forms from full letter to short note to card to e-mail to verbal, either at the time of presentation or the next time I saw them or diliberately going out of my way to see them to say thank you. I also have numerous times not given a thank you in any form. I similarly try very hard not to be concerned with whether I receiver a thank you or return gift when I give a gift because it is important to me for my gift to be a gift not an obligation to them.
For the thank yous I give, I try to go by what my true feeling of thanks is. There are some gifts that are overwhelming and some that are amazingly underwhelming. I'm not speaking in a financial sense but in a true sense. A $2 used book on a subject I've been searching for a good book is a much better gift than a $50 gift card for a music store (a $10 version of which I got every year from an in-law family gift exchange no matter who I was getting the gift from in the lottery), particularly considering I don't listen to much music.
I guess my overall point is that I try to avoid any rules on the subject of gifting in general (as much as I can get away with anyway) either in giving or receiving. Rules create obligations and obligations are not gifts. However, I DO focus on being a generous and giving person, but not on following any arbitrary/societal rules.

Bradamante, the problem with threatening to send the gifts back is that it is likely to end up reinforcing the perception of an arbitrary hoop rather than teaching a true spirit of gratitude -- especially if the emphasis is placed on the production of a letter-perfect handwritten note in ink. With so much emphasis put on the mark-making process (and insisting on using the most frustrating possible option of those available to a twenty-first-century person) rather than the emotion that's supposed to be evoked, the production of thank-you notes often becomes an exercise in exasperation instead. I think children (and adults) would be more likely to actually write thank-you notes if the rules about the mark-making process were relaxed and they were allowed to let small errors in spelling slip past, were allowed to use a pencil (which can be corrected on the fly), or even allowed to use modern writing technology. I honestly don't see how a note produced by the marking engine of a laser printer should be considered automatically less grateful than one produced by pushing around an instrument that trails colored liquid. If even cool script fonts are considered too cold and impersonal, at least let the poor soul use a graphics tablet and stylus and a drawing program like Illustrator, so they can correct errors instead of having to laboriously write the note over and over again until it becomes a hateful chore.

On a side note, the response to "Thank you" (salamat) in the Philippines (Tagalog) is "walang anuman". This also is best translated to "It's nothing".

Leigh, you sound like one of my kids. I tell them if you don't write a note, then I'll send their gift back. Luckily, so far they want everything so it hasn't been a problem! Courtesy is worth the trouble.

Don't complain too loudly about people giving you books you disagree with. Look at it as an opportunity to give them a good Catholic book.
I was always a little disappointed that my conversion to Catholicism did not bring more reaction from my family.
Had they argued against it, that would have provided me a great opportunity to evangelize.
As it is, I have to look for openings where I can.

Annalucia, the problem with not writing a thank you note for an insulting or inappropriate gift is that the giver knows by your silence how much the gift annoyed you--which will probably encourage them to continue. May I suggest the following?
Dear Sister-in-law,
Thank you so much for your gift of the book titled "Leftoid Rant." I always appreciate being able to read the views of those to whom I am most opposed politically; this makes it so much easier for me to formulate arguments to be used against them both in active politics and in casual conversation. I have also enjoyed the fact that "Leftoid Rant" has confirmed my suspicions that many of those on the other side cannot write coherently, pay little attention to the rules of good grammar, and are completely incapable of formulating logical thoughts. None of this would have been possible without your gift, since I would never have purchased this particular tome for myself.
With grateful appreciation,
Signature.
With such a letter as this, the burden of annoyance has been placed where it belongs: on the shoulders of the inconsiderate and even rude giver of inappropriate gifts!

A lot of people today regard a handwritten thank-you note as an arbitrary hoop to be jumped through, rather than a genuine indicator of gratitude, and really don't see why longhand on paper should be regarded as more "real" than e-mail, or verbal expressions of gratitude. Especially when one has a large number of such notes to write, with most of the words in each being the same, having to hand-write each of them often seems like busywork rather than any real metric of gratitude.
Part of it is a general decline of the role of handwriting as a means of interpersonal communication (as opposed to casual jottings to jog one's memory). Many people don't develop good handwriting skills in schools, so being expected to turn out a letter-perfect hand-written thank-you note becomes a monumental chore, to the point that any sense of gratitude becomes lost somewhere in the seventh or eighth attempt that has to be abandoned due to a dropped word or suchlike trivial error. Little wonder that a lot of people resort to scribbling something semi-legible and semi-coherent on a little card.

Sorry, my eyes were crossed, it was Robin, not Tim.

Annalucia-
I submitted a question similar to yours to the Ask Emily section of the Emily Post website. I am curious to see what they will say- not that they are the final authority, but ettiquette is supposed to be their area of expertise. I'll let you know if they reply.
On the one hand, I would agree with Tim if you are talking about a tasteless or thoughtless gift, but one that is meant to be insulting or attacking of your personal beliefs might be a more serious "offense". My brother, a non-denominational Christian, has given us books that deal with different subjects approached from his perspective, but what if it were something blatantly anti-Catholic? Does it still deserve a "thanks for the thought" when it is pretty clear what the thought was?

Oh, the guilt!!!

Annalucia, I have had the same problem. I usually write a thank-you note but restrict myself to thanking the giver for "thinking of me." Since I am usually effusive about gifts I like, people seem to get the hint!
(I am in a family that considers failure to write thank-you notes an almost unforgiveable sin, so an evasive thank-you note is far better than no note at all.)

What about tasteless or deliberately offensive gifts (such as a book by a ranting Leftoid, from my sister-in-law who shares their political views?) I deliberately do *not* write thank-yous for these, in the hopes that it will discourage the giver from giving me anything else, ever. If anyone has a better solution, please let me know what it is.

From the Emily Post Institute website:
Is it necessary to write thank-you notes to family members?
If you’ve thanked someone in person for a gift, a thank-you note isn’t obligatory. But, it’s never wrong to write a thank-you note. If you receive gifts from family members that you won’t see to thank in person, write them a thank-you note—both to let them know their gift arrived and that you liked it.