A link to the various collections of Leonardo's drawings.
Leonardo is most known as a "painter" even though we only have about 20 that have survived. That is not as true of his drawings - which we have thousands and can only imagine how many he originally sketched.
Leonardo used his sketching skills for far more than just doodles but as a scientific tool. To detail the human anatomy with the precision of an MRI or x-ray machine. If there is a single thing that gives us the biggest glimpse into his mind it's through his sketches. As he said the image of a thing is more accurate than any words. He used his talents to draw designs for war machines, a mechanical calculator - a tank, air planes and these designs are able to relay information at a glance that would require an encyclopedia to understand any other way.
The most impressive feat for a designer / inventor/ creator/ artist is to be able to relay what they intend to relay in the shortest possible way. If you imagine trying to explain what an airplane was and how it worked to someone who never even conceived of flight before - you would have to use words that they didn't know - and that were most likely not even spoken. Those issues could be easily eliminated by using images - which at the time was limited to the skill of the artist but thankfully Leonardo was and still is one of the best sketchers in the world. We can learn more from his drawings and designs than his words - at least to an extent. There are still some ideas that are hard to translate visually such as gravity or inertia or evolution - ideas that take more abstract symbols and knowing the meaning of images that are not to be taken literally. This aspect to many of Leonardo's drawings imbue into them a sense of mystery that seems dark and shady - but only when considered in the context of the time they were created. His secrecy and obscurity are the results of the need to mask his ideas from ignorant and dangerous minds of his time - but not our own. The ideas he was hiding are not common knowledge but no less complex or significant.
Perhaps the most interesting thing about Leonardo's style is it's combination of individuality and universality. If you think about drawing something in front of you - the aim could be to draw it as accurately as possible. So that if you hold up your drawing next to it they look as similar as possible and if you were to show it to someone they would recognize it for what it is because it looks like it. That is one skill set and type of drawing - the other is stylized or subjective.
That leads to a type of confusion because we may not be able to even recognize what it is on the paper let alone what it means. That doesn't mean it has no meaning or even that it is being purposely secretive - but that we do not understand that visual language. It's comparable to playing a game of pictionary through time - imagine not being able to use any modern examples such as light bulbs or computers or symbols. Then imagine the different types of culturally exclusive things like political holidays or family traditions that no one else would understand. Then finally imagine that you are 500 yeas ahead of your time and you have to literally make up your own visual language to convey inventions and ideas that had to at the same time be obscured and obvious.
That is the challenge in studying Leonardo's drawings not only do we have to find "waldo" we have to ask ourselves "What's waldo?" - Who is em, and what does it mean? The other challenge is being able to distinguish fiction from fantasy and truth and lies and some things are not black and white but sfumato.