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С артиклями беда: практически все не на своем месте, и ни одного там, где он должен быть.

I was often grinning/chuckling, not smirking while reading the book. Smirk - это нехорошая ухмылка.
Either - это выбор из двух, а не A, B or C.

I know this book is spin-off - нужен неопределённый артикль перед spin-off.
There is the little glossary - а вот тут артикль не нужен.
before the main reading - наверно, лучше было бы before reading the main text.
But at other hand - обычно тут предлог on.
there was the entire exhibition - не уверен, что слово exhibition тут к месту.
I try too hard to answer the question - сдаётся мне, что тут должно быть прошедлшее время (I tried too hard, а лучше того very hard).
I can't resist - опять же по ходу тут должно быть прошедшее время.
this is a very funny - не нужен артикль.

Не худший вариант. Не "Рунглиш". Адресат должен понять.

Что бросилось в глаза:
"But the jokes aside" - лучше "Aside the jokes"
They beaconed me - They BECKONED me? Тут уместно They attracted me, to beckon - это что-то в духе "манить пальцем".
especially useful are those to express my displeasure - это не по-английски.

Более внимательно, в том числе соответствие артиклей, не смотрел. В ответах ниже даны верные замечания.

Thank You, Mimi, for sending me A free signed copy of the book and THE bookmarks TO BOOT.
I know this book is A spin-off of the series "Accidentally Yours", but I DIDN'T HAVE any difficulty reading it as A standalone.
There is A little glossary and A description of characters INCLUDED at the end of the book, WHICH I found quite useful to PERUSE before reading THE BOOK PROPER.
I COULDN'T HELP CHUCKLING while reading the book, AS IT ABOUNDED WITH funny moments and phrases.
THAT SAID, I FOUND THE WORD "kolbas" (the plural form of the WORD "kolbasa") USED FAR TOO MANY TIMES FOR MY TASTE.
ON THE other hand, there was AN entire exhibition to satisfy the ying-yang proportions, WHICH FILLED THE book WITH THE SENSE OF cosmic balance and harmony.
I like to FIND out new things while reading.
THE THINGS I FOUND in this book ARE as follows:
Jackalope and THE tree octopus,
A bunch of old Mesoamerican gods.
A FEW new words TO CATER TO my non-native English. Especially useful WERE those EXPRESSING displeasure.
I really ENJOYED the final chapters: So, Sadie WAS spoiler-spoiler-spoiler-spoiler-spoiler-spoiler.
Wow, THAT WAS AN UNEXPECTED twist!
AND I AM A FAN OF unusual twists!
THE quiz PROVIDED in the book WAS unfair THOUGH. I WAS TRYING HARD to answer the question CONCERNING clowns, but it always ended IN a fit of coulrophobia and I COULDN'T understand WHICH ONE OF THE THREESOME: A, B or C WAS THE SCARIEST. The KEYS AT the end of book DID LITTLE TO CLARIFY THINGS.
The book PROMPTS ONE to AWAKEN ONE'S CREATIVITY. The spotlessly white pages MESMERIZED me, AND I COULDN'T HELP BUT TRY AND make MY contribution TO THE author's collection of unicorns.
MEET MINKY THE UNICORN
But, jokes aside: this WAS a very funny and nice read.