As video games grew more complex in the 16-bit era, we started to see game guides becoming more popular. There were companies like Squaresoft that profited from selling guides for their RPGs (like the Final Fantasy series), and it's often been thought that they included some secrets that were impossible to work out on your own, just to sell more guides.
It turns out that game guides were incredibly popular in Japan during the 8-bit era, even though games were far simpler, as a guide for the original Super Mario Bros. for the Famicom outsold every other book the year of its release (via Magweasal).
The best-selling book in Japan in 1985 (that wasn't a manga) was Super Mario Bros.: The Complete Strategy Guide, which gave tips and tricks for beating the first game in the Super Mario Bros. series.
The increasing popularity of the Famicom in Japan led to Super Mario Bros.: The Complete Strategy Guide claiming the best-seller spot for a second year in a row, as gamers were clamoring for information on how to conquer all of the levels, as well as information about the mysterious Minus World glitch.
If you are curious about the quality of such a popular game guide, then you can read it yourself, as Super Mario Bros.: The Complete Strategy Guide was translated to English and released overseas under the name of How to win at Super Mario Bros. (via Magweasal), which was distributed via early issues of Nintendo Power.
The strategy guide business is practically extinct, thanks to websites like GameFAQs offering all of the information you would ever need to complete any game. There are also places like Reddit, where you can receive quick answers to any questions you have about any specific game. You could even just put a message on your Facebook or Twitter page asking for advice on a new game, and some helpful nerd will probably be around to lend you aid.
The lack of Internet in the '80s meant that fans had to resort to game guides in order to complete tricky games, which is how the original Super Mario Bros. was able to form the basis for a best-selling book in Japan.