there was a study saying that there is not “the” best way of learning, but it is best to combine multiple ways, like with an app, by book, listening to audio only (i listened to radio stations via internet and got some exercise for free), a bit of talking, visiting a country that only speaks that language and so on. trying everything a bit in parallel.

that is because of our brain learns better when given more different types of “connections” to learn.

i started with duolingo (website only, not the app and only the free parts) 4 years ago and now i speak quite fluently. but i also partly read a book about grammatics, visited a spanish speaking country (more than once), viewed movies with only subtitle in my language and did lots of phone calls in spanish only.

my advice is:

look at free apps, whatever pleases you, take chances, listen to the sound (movies, radio), try to speak, and read easy books or go through exercise books.

duolingo is good to keep on going while not really motivated as the shortest thing that counts are really only minutes and one can choose to do something that is already easy. this way at least continuation is kept even if pace is down for a while. and it is much easier to go on with pace when not having really stopped.

i have to admit, that my point ‘just don’t do it’ in reality does not garantee to prevent any trouble. it still is possible to be sued for things someone else did.

also one suggestion to think about:

if the seller just sprays some random changes over a book for every sold version, one would have differences in “every” sold version to every other sold version. by blindly changing those parts to something else you could reveal which exact two/three versions you had for diffing.

UPDATE: someone else here had the same thought a bit earlier…

my suggestion to not do it stays the same ;-)

it could be interesting to figure things out how they work, what could be done to prevent or circumvent such prevention, but actually doing it seems risky no matter what.

have a look on “snowdrop” (search together with “steganography”), its basically the opposite of what you want, but worth mentioning here. watermarks could be placed into whitespace (not limited to actual spaces or linebreaks, intentionally changed usage of paragraphs, tabs or even page boundaries could possibly be detected after scanning andeven after OCR. IMHO snowdrop uses -depending on choosen operation mode- small errors like misspelled words, commata etc but also has a mode that comes along with fine grammar and without misspelled words…

how do you make sure that by diff’ing two versions you do cover "everything’ that has been deliberately placed into both documents but share literally the same informations?

lets say you bought two books at two different stores with two different watermarks. if the watermark contains the date and time of the purchase and the only difference of this were the minutes because you bought them within the same hour, the remaining watermark would point to all buyers that bought exactly this book in this hour - worldwide. but still it could be “very” precise depending on all other(!) buyers, if they exist at all within that timeframe. what if the watermark includes unix epoch? then the part which is the same in both watermarks would not be bound by hours, but by seconds, 10seconds, 100seconds etc.

and you could not know if there were other watermarks hidden that just happened to be the same for your two (three.?) purchases (same country, continent, payment method, credit card holder name, name of internet provider used during purchase, browser used etc.) it fully depends on the creator of the watermark what would be included and what not. if you happem to know all that (without any possibleexemptions) you might be on the safe side, but if not…

my general suggestion here is:

• if you want to be sure to not getting into trouble, then just don’t do it.
• if that book is too expensive compared to its content, just not buying it possibly also helps the market to fix the problem.
• save that time and instead help those who already fight for a better world.
• search already licence free books (or such as “cc” licensed) and promote those instead, help improving free resources like openstreetmap, wiki* but do not publish licence-poisoned content there, wtite it yourself, alway.
• write your own book and publish it free.

just to mention… the “safe” side sometimes seems limited but maybe is actually not, if you really look at it.