By far his most popular book is "It's Kind of a Funny Story". I think this is because it's one of the earlier contemporary works to speak candidly about the mental health issues of teenagers. I know it's not the only work to address those types of issues, and nowadays that subject has almost become it's own genre, but Mr Vizzini's style is so blunt it seems to have resonated with a lot of people. Like, he's not trying to sugarcoat anything and you feel like he's being real with you about the whole experience.
In all honesty, I think his style is something that's not gonna resonate with a general audience. When the author refers to the story as "Kind of a Funny Story" he really means the "Kind of". The humor is situational and how it lands is gonna depend on whether or not you can empathize with what the characters are going through.
Mr Vizzini's work can make you laugh, it can also make you cry. It can make you feel uncomfortable, it can make you think about your life, it can break your heart. So this story is kind of a lot of things.
Its about a kid who wants to commit suicide and his journey through the mental health system.
Full disclosure: I work in healthcare, so I've seen firsthand a lot of what the main character experiences in the story. It wasn't as much of a revelation for me as it will be for some people, but because I felt like it portrayed it pretty accurately I think it gave me a greater appreciation for, like, what the patient is going through when you're caring for them.
Also I'm making this post from memory: I've read the book twice and watched the movie twice.
Overall
Book-
I think the book definitely tells a fuller story of what people go through when they enter the mental health system. It goes into detail about this kid's life before he got institutionalized and explores all the different factors that led to his predicament: even going into detail with the bits that aren't so savory like his sexual frustrations and his drug use. (This is the part of the story where when I read it I realized the author's being real with his audience. It's not the perspective of a teenager telling adults what they want to hear, this is actually how he feels about sex and how he feels about getting high. It's surprisingly nuanced and analytical. He's not moralizing the issue: there are positives and negatives to everything.)
Movie-
The movie is a lot more streamlined. It starts when the kid asks to be admitted and has these elaborate flashbacks to his life before.
The movie also cuts a lot of the potentially objectionable content. I generally think of this as a good thing because it puts some important issues out there in front of more people. Families could watch it together and use it as a springboard for discussion about mental health issues and concerns. (Though I doubt most kids would get very much out of it without adult guidance.)
Perspective/Narrative Style
Book-
The book is written in a first person POV and it has this kind of a candid, informal style. When the main character relates what someone else said, he often prefaces the quotation with a phrase such as 'so-and-so was like' or 'so-and-so was all'. It conveys the sense that like maybe he's just paraphrasing what the other person said from memory and it's not all word-for-word exactly what happened.
Movie-
I would assume a visual medium would necessitate a more definitive depiction of what characters in a story said or did, but interestingly the movie ends up doing something similar to the book in this regard: when the main character thinks about the past, it's pretty clear a lot of the time he's kind of fantasizing and that what actually happened wasn't as dramatic as he remembers it. Its used to try and inject more humor to the story and sometimes it lands well.
Characters
Both Book and Movie-
At one point in the book, the main character is described as having a flat affect, which if you're not familiar with the term it basically just means that he doesn't project a lot of emotion in his face or voice. The actor for that character in the movie really nailed that aspect of his character with limited facial expressions and a deliberate, dry style of line delivery.
What the movie doesn't portray as well is the character's internal monologue. Some of this is conveyed as like the main character talking to the audience, but there's also this really interesting thing where the main character has an imaginary drill Sargent voice in his head who he has conversations with. It adds a lot of color to the narrative, and establishes the main character's metal mindset as it's own kind of domain with it's own rules and processes.
Understandibly, this would be difficult to establish in a visual medium, and the movie doesn't even try. Unfortunately though the absence of this dynamic, coupled with a slow line delivery by the actor, makes it seem like the character is slow to process things (which is definitely not true of the book version of the character). In the book it's rather it's more of the opposite: like he's thinking to fast and can't express it because 'It's all too much' and he gets overwhelmed.
For example, when someone asks him about his hobbies:
'I work, Monica, and I think about work, and I freak out about work, and I think about how much I think about work, and I freak out about how much I think about how much I think about work, and I think about how freaked out I get about how much I think about how much I think about work. Does that count as a hobby?'
^^What an egregious bit of word vomit, right?! And he doesn't SAY any of it! It's all just what he's thinking in is head, wishing he could've said in that moment.
Aside from the main character, there are a wide array of colorful side characters from all walks of life. I think they're depicted delightfully in both adaptions. In the film some of these people are combined into a single character: the most obvious is the one played by Zach Galifianakis. I kind of liked this, because it gave more interest to the character and you're low-key following him, hoping he can sort his shit out and be involved in his daughter's life.
The downgrade with the film is that you get to know fewer of them. One character I felt was really short-changed was Muqtada. When he finally gets out of bed in the book you feel what an achievement it is for him and you're just so happy for him, in the film it just doesn't have the same affect.
(Also in the book Dr Minerva is the main character's regular therapist. She's not, strictly speaking, employed by the hospital. He has to wait for her to make rounds.)
But the one character change I found most irritating was Noelle:
(LIKE WTF IS THAT!? THAT'S NOT A SCAR THAT'S SOME POORLY APPLIED MAKEUP!!)
It's a total injustice to her character. I could get not wanting to glamorize self-harm, but IS THAT NOT WHAT YOU'RE DOING WHEN YOU MAKE SCARS LOOK AS PRETTY AS THIS??!!!
Instead of presenting her journey as this thing like: 'If you're open about your struggles you can help other people though theirs and form genuine connections regardless of what you look like'
It becomes: 'Well you're not really that ugly. People can almost forget you actually have struggles at all.'
And that's friggin insulting!
Conclusion:
I recommend the book if you're a teenager or an adult. I recommend the movie if you're a family with younger kids who you feel comfortable introducing these types of issues to. I can't promise you a great time with either of these options, but I do think it will give you some food for thought.