Best Chess book for 1200

Maybe: Winning Chess by Irving Chernev and Fred Reinfeld (1948)
https://web.archive.org/web/20140708093415/https://www.chesscafe.com/text/review919.pdf

I would not recommend the book by (I think) Susan Polgar's dad. Supposedly it's problems he used to teach Susan Polgar and her sister, but there was so many queen sacrifices to achieve mate, I got tired of the book... and I still, didn't run out of the queen sacrifice tactics.
It's a mammoth book for sure. I think it was something like 5000 problems, but don't quote me. I think it's been out of print (hopefully) for years.

Many books that fit the category recommended here...
Good Chess Books for Beginners and Beyond...
https://www.chess.com/blog/RussBell/good-chess-books-for-beginners-and-beyond
"Winning Chess" by Chernev & Reinfeld is certainly an excellent tactics book recommendation, follow-on to the Bobby Fisher book.
Other very instructive books focusing exclusively on tactics and employing a puzzle format are...
"1001 Winning Chess Sacrifices and Combinations" and "1001 Brilliant Ways to Checkmate", both by Fred Reinfeld. Nothing but puzzles - lots of them! With answers in back of book.
"Chess Training Pocket Book": 300 Most Important Positions and Ideas by Lev Alburt. Highly instructive tactical scenarios and motifs, presented in puzzle format, which every chess player would benefit by working through. I highly recommend.
"Winning Chess Exercises for Kids" by Jeff Coakely. Despite being littered throughout with kiddie cartoon characters in an attempt to market the books to kids, the book's content is not only for kids. In fact, there is a lot of instructive content, mostly in puzzle format, for any improving chess amateur, especially at the beginner-novice level.

I would not recommend the book by (I think) Susan Polgar's dad. Supposedly it's problems he used to teach Susan Polgar and her sister, but there was so many queen sacrifices to achieve mate, I got tired of the book... and I still, didn't run out of the queen sacrifice tactics.
It's a mammoth book for sure. I think it was something like 5000 problems, but don't quote me. I think it's been out of print (hopefully) for years.
5334 Problems if I remember correctly. I don't have it but I remember flipping through it. You are right, it is massive and heavy.
".... Kasparov's Predecessors book is simply brilliant, but way too analytical for beginners. ..." - IM pfren (2020)
https://www.chess.com/forum/view/for-beginners/as-a-beginner-should-i-study-older-chess-games
"... a lot of deep analysis has gone into [Positional Decision-Making in Chess] ..." - GM smurfo (2015)
https://www.chess.com/blog/smurfo/book-review-gelfands-quotpositional-decision-makingquot
"... a book rich in material for players who already have a good grasp of many middlegame and endgame ideas. I believe also some material ... will need to be read more than once …"
http://georgiachessnews.com/2015/09/01/review-positional-decision-making-in-chess-by-boris-gelfand/
https://www.qualitychess.co.uk/ebooks/PositionalDecisionMakinginChess-excerpt.pdf

How to Reassess Your Chess, 4th Edition by IM Jeremy Silman may be just a tad more advanced than what you are looking for (the introduction indicates the book is aimed at 1400 - 2100), but it might be a book to consider, if not now then in the near future.
I think GM Yasser Seirawan's books, Winning Chess Tactics, Winning Chess Strategies and Winning Chess Endings would be great books to add to any chess libraries. Let's face it, most books on tactics, strategies and endings pretty much cover the same material. What makes them different is the writing style. GM Seirawan has a great way of explaining things.
You will always learn more from a book that is easy to read and easy to understand, and that is why I highly recommend Seirawan's books - they are enjoyable and easy to understand, making them so much more useful.
https://www.silmanjamespress.com/shop/chess/how-to-reassess-your-chess-4th-edition/
https://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner
https://web.archive.org/web/20140708090229/https://www.chesscafe.com/text/review492.pdf
https://www.chess.com/article/view/book-review-winning-chess-endings

Thanks again guys! Since reading Bobby Fischer Teaches Chess, I have bought a couple more books, one of which being How to Reassess Your Chess - Jeremy Silman. Although I could tell it was a great book for improving one's game, I found I was a little out of my depth. I could read through it fine but found it difficult and not as enjoyable. Hence why a 'continuation' of Bobby Fischer Teaches Chess aimed at a slightly higher level would be perfect for me - I look forward to working my way through 'Winning Chess'!
"How to Reassess Your Chess, 4th Edition was designed for players in the 1400 to 2100 range." - IM Jeremy Silman (2010)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
Might want to consider Amateur's Mind by Jeremy Silman.
https://web.archive.org/web/20140708094419/http://www.chesscafe.com/text/ammind.pdf
https://www.silmanjamespress.com/shop/chess/amateurs-mind-the-2nd-edition/

At 1200, it probably isn't your understanding that needs work. Rather you probably need to work on your skill, which means tactics.
However, there are several ways to do this. You can raw solve a ton of tactics. This will improve your calculation significantly, and help build pattern recognition.
Alternatively, you can work through a book like the woodpecker method (https://www.amazon.com/Woodpecker-Method-Axel-Smith-Tikkanen/dp/1784830550). The first run through will also help your calculation significantly, but additional run throughs will make your pattern recognition immensely better.
I used this technique with a similar book for about 3 months and went from about 1400-1800 shortly thereafter.