Ah, advertising. Trying to grab eyeballs. Create an impulse. Make a sale.
Book trailers. 30-sec video clips. GIFs. Animated ads. Do they sell more books than a static, flat ad? Are they worth the money? The time to create?
Guys, hehehe, ~slaps knee~, let's have a reality check. If sales and marketing divisions knew without a doubt what kind of advertising would guarantee a sale, there'd be a revolution in the industry. 85% of S&M (yes, yes, snigger if you want) is spaghetti against the wall. When an ad has a conversion rate of greater than 10%, it's considered a rousing success. A good campaign is somewhere around 5%, the average is 2%.
The holy grail of advertising is a viral campaign in which the company doesn't have to do much. A Taste Maker has fallen in love with the product and given it legs. If they can't get a Taste Maker, get a teenager (lookin' at you, Fidget Spinners). They're the second most influential group. Women are the group with the greatest purchasing power. You place your ads where they'll be seen by influencers and purchasers. 'Dems da broad strokes of S&M that haven't changed in decades.
So, what's that have to do with trailers, clips, and GIFs?
Disclaimer: I don't work for a publisher or an advertising agency. I do not work for an ad farm or a research firm. I do not have actual stats on campaigns. I do have amazing Google-fu and experience trying to get people to buy stuff they don't know they need.
Let's start with book trailers. You don't make them for the audience you wish you had. They're not a sales tool. You make them as a reward for the readers you already have. At best, they're a retention tool. Usually, book trailers are done because the author simply likes having them.
But what if I'm advertising on YouTube and I want a pre-roll or in-play video ad? Then you probably are spending too much on your advertising budget. ~cough~ In all seriousness though, you have 10-seconds to grab the attention of your viewer, by 15-seconds you've lost 2/3rds of them. 30-seconds is your max time allowed for the ad. My gut says the ROI isn't there for an author. You're selling a book, not a Marvel movie. YouTube isn't where you'll find your audience. If you're committed to a video longer than 60 seconds, you're likely better served by having your own channel and directing existing readers there--again, for the fun of it, not the sale.
Viral Video Clips (15-30 seconds): Here's the thing, videos take time to render on the page. On mobile apps, they can be turned off, aka, never seen. They're usually prioritized to render last. By then, the consumer's already clicked away or they're reading the content of the page and your video will piss them off when it finally plays because it's now a distraction and it's probably stolen focus (e.g. claimed the cursor's control from scroll bar).
However, if you have fans who send clips to you of your book in the wild, of their reviews, of cosplaying your characters, etc., my friends, that is AWESOME. Promote that stuff on your website (after getting the reader's permission). Do not make them into ads.
Flash(y) Ads/HTML5 Animated/Rich Media: When the tech behind Flash ads was new to market, it was something hungry advertisers and hungrier ad sales teams pushed. To the point that consumers are now as blind to them as static ads. If you have your heart set on a rich media ad and some cash to burn, limit the play to 6 seconds. More than 9 seconds and your Click-Through-Rate (CTR) tanks. 6 is the sweet spot.
6-second ads, aren't those Vine video clips? Folks, Twitter killed Vine. It didn't replace it as the best tech, the company Twitter bought Vine then killed it. Pretty much tells you everything you need to know. Yes, there are other providers out there. Yes, of course, the tech is still used. If it could have been successfully monetized, it wouldn't have slunk off to the shadows.
GIFs: As a file format for advertising, GIFs aren't typically supported. It's most likely you're thinking of using gifs on your website or on your social media accounts. On sites that support the playback, it's not the time that's limited, it's the file size. It's technically possible to make a 3-min gif. Use GIFs as you would any status update on your social media --to engage with your followers. If you deploy these as cheap/free sales opportunities, you might as well be screaming BUY MY BOOK at your audience. That's how you lose followers.
There is nothing wrong with static ads. Vendors have all kinds of limitations on text versus image and what kinds of images are not acceptable, but there is an industry standard for visual and file size. One ad can go in a lot of places and across a lot of platforms. For now, static ads are still the best bang for your buck.
A better option for your time with minimal money: Build your newsletter subscriber list. Send free short stories to your subscribers quarterly.
The best option for your time and money: Write the Next Book.