I guess you'll find it after passing the cave. which means you must have found all 6 companions.

One hint for additional books. don't remember how exactly I found them, but I guess it was about Facebook: did I mention that I hate facebook? I absolutely do, I never will create an account. No - never, really, I won't change my mind...

[EDIT]
On second thought. Maybe you already have them. I can not find the button where I got those hidden books anymore. So maybe the developer has removed it as your character already starts with 3 books.
Sorry for a misleading post.

"Victory is mine!"

The button disappeared once I got the 3rd book.

Without any direct spoilers:

The monument will be on the main map. You can't miss it once you have the book equipped.

Now the monument by itself is useless, but you can enter some codes there to get rewards. How to get the codes? They are hidden in some battles and in some cutscenes. They could be anything from the shape of the map to a small picture in the background.
You don't need the book to find the codes. However if you have the book equipped then at the start of the battle/cutscene you'll hear a sound effect that will tell you a code is present in this map/cutscene. So once you find better books feel free to equip them instead. Basically you can equip the book of secrets once, see if there is any sound effect at beginning of map, then replay the map with better books equipped.

You can get books from bonus battles.

If you want my opinion don't bother too much with the book of secrets. It's fun finding those codes but not strictly necessary to finish the game. What you could do is once you finish the Normal game, go secret hunting and find all codes.
There is a website that gives incremental hints for the codes.
Also unlike the bonus battle books, some of the secret statue books might feel a bit overpowerful. Consider only using them in the last extreme battles of New Game + when you need all the help you can get.

Do the main storyline. And from time to time try doing the advanced/extreme/bonus maps. If you find them too tough just continue with storyline.

Is the tone blatantly obvious when it plays (assuming volume is decently high)? I had the sound very low yesterday, since I was playing while other sound sources were going off, so I might have missed it. Do you remember which map is the first to have the sound play (I want to know what I'm listening for)?

Is the monument a map location, like the battlefields and towns, or do I need to click on it without actually walking to it (or being able to walk to it)?

The monument is a map location. And you'll see it always when you euip the book. Even if you go past the area which branches into the location.

I don't remember exactly the earliest map with a secret code. However there is a code hidden in... Tips.

Equip the Book of Secrets then from the main map click on "Tips". Note the sound and the red flash on top left. That's what you should look out for, though the flash might be of a different colour.

I don't remember, but could it be that the Book of secrets is only active after the player found the monument?

One doesn't only hear a sound, there is always a little light blinking in one of the upper corners of the screen. [ZFR already mentioned that...]

I guess I stop helping today. If got a very bad headache...

Are all clues visual? Is there anything text-based by any chance?

I'm sure larsiusprime would love this game it if you sent to him.

The one in Tips is text based. Not all are visual.

Could you please tell me what it is? In PM if you don't want to spoil it here?

did zfr reply? i am online, so I could take over this job.

Please excuse my typos. the keys of my keyboard always get stuck.

Thanks. I'll make sure the book is equipped and see what pops up. If nothing comes up, I'll keep going until the monolith shows up.

For those replying, I'd prefer no spoilers, if possible. Just my personal vote, and thanks! :)

Already solved. I was just wondering how text-based hints would work out for those who don't play the game in English. As it turns out, it is an issue.