Bought a novel in my target language and stopped on page four: when are you actually ready to read real books?

The book was too hard, and it is the standard mistake. Nothing is wrong with you and reading does work at your level, you picked the wrong first text.

The rule that matters: you should understand most of it without looking anything up. Roughly one unknown word every few lines is the range where reading is productive, because you can infer from context and keep momentum. Thirty lookups on a page is not reading, it is decoding, and no amount of it builds fluency.

So the path in:

Graded readers. Books written for learners at defined levels. Unglamorous, and they are how a lot of people actually make this jump. Start below where you think you are.

Children's and young adult books in the language, which are the usual next step. Note that children's books are not always easy: they often use unusual vocabulary and playful language - so young adult is frequently the better bridge.

Something you have already read in your own language. Enormously helpful, because you know the plot and can carry meaning across the gaps.

Non-fiction on a subject you know well. You already have the concepts and half the vocabulary.

Then a normal novel, later, and it will feel unremarkable when you get there.

How to read once the text is the right level, because technique decides whether it becomes a slog again.

Look up far less than you want to. A rule that works: only look up a word if it appears repeatedly, or if the sentence collapses without it. Otherwise keep going. Most words you skip you will meet again, in a context that teaches them for free.

Read for the story, not for completeness. Not understanding some of it is normal in your own language too and you never notice.

Read in longer stretches rather than in careful minutes. Momentum is doing real work - your brain gets into the language and starts predicting, which stops happening if you break every sentence.

Use an e-reader with a dictionary if that reduces the friction enough to keep you reading. A tap is much less disruptive than a phone.

Do not translate as you go. If you find yourself producing an English sentence in your head for each line, the text is too hard.

And about retention: you are not supposed to remember the vocabulary from one encounter. Reading works by exposure over volume, not by extraction. If you want words to stick, note the handful that were genuinely blocking and put those somewhere - not thirty a page.

The technique that unlocked this for me and that nobody suggests: reread.

Take something short and slightly hard: a chapter, an article, a short story - and read it three or four times over a week rather than reading four new things.

First pass: slow, some lookups, mostly getting the shape.
Second: noticeably easier, fewer lookups, you start noticing how sentences are built rather than what they mean.
Third: close to fluent, and this is the one that changes something. You are reading rather than decoding, at your actual level, which is an experience you may not have had yet in this language.

That third-pass feeling is the thing to chase, because it is what reading is supposed to be, and constantly moving on to new harder texts means you never reach it.

It also builds vocabulary better than the same time spent on new material, because repeated encounters in context is exactly how words stick.

On your paperback: keep it. Put it away and try it again in six months after a dozen graded readers and some young adult books. The version of you that opens it then will have a completely different afternoon, and finding that it has become readable is one of the more satisfying moments in learning a language.