I actually bought and read quite a few Instrument books in preparation for my CFII check-ride. I also went to a big name college which focused on a few other titles (material... like most things in aviation training I found hasn't seen significant updates since the last 8 years of reading it)
Disclaimer : I haven't read the Instrument Flight Training Manual so I will not comment.
Not Recommended:
I have read the Instrument Flight Manual from Kershner, and although I really like his other books wasn't my top recommended book. AT ALL COSTS: Stay away from the Jeppesen Instrument/Commercial book! UNLESS it is a guided discussion ground school, for self study its a no-no! (very personal opinion, forced to read it in college and felt really disgruntled). Jeppesen content wise is by far the LEAST helpful but my goodness the pictures are very pretty! I use it for show and tell, but I reference a book below for material. Rob Machado's book was a bit TOO silly and TOO simple to really make it a solid go to for me.
Recommended:
For knowledge The ASA Pilot's Manual - Instrument Flying PM 3 is by FAR the best all-in-one for Ground self-study. It covered everything I needed to study for the Oral and Knowledge tests. Word of caution, the book is 640 pages, all of them useful (even dedicated chapters for TAA aircraft). If your ground school has a different book, it still is a great read to give a dead simple easy to read understanding. However, one thing the ASA book lacks was a solid IFR weather section, covering all of the products that are out there (and even a few that aren't -LI/K Index). The ASA book covers the basics of weather theory, but the FAA book is the best for specific products.
AS ALWAYS:
The FAA books (including AIM) are by far the BEST for specific content, although they aren't the best at getting to the point, or sometimes get too far into the weeds (like most government material - I'm in the government so I have a developed an immune system)
I would recommend to follow Ron's advice...PLEASE do not fly without an Instrument Instructor if you are serious about getting the ticket done efficiently. There are some NASTY habits that we have to work out that ultimately cost you in the end. That isnt just personal advice, thats advice I have picked up by some of the best in the industry.
For Computer programs, I prefer the pricey Sporty's...but nothing really beats a solid Person-to-Person Instrument Ground School.
Study before flying...its not like the PPL where you can easily pick it up as you go. The more you are armed with information, the QUICKER your instrument understanding will click, and you'll be shooting practice no-gyro approaches to simulated minimums in no time!