Bookshelf vs diffuser panel. Who wins?

@ditusa However, take everything Acoustic Fields says as an attempt to separate you from your money.  I spent $150,000+ on constructing a listening room with the contractor who worked with him on other projects.  He never supplied the plans, they were in his head.  I had to pay by wire.  He put me off several months, then shipped incorrect sized carbon filters, too many acoustic filters boxes, ordered too much acoustiblok rolls and incorrect sized ceiling acoustic filter boxes.  After alerting him that the front and back wall box filters were killing the sound (overdamped), he said send me another $30K for front and rear quadradic diffusers.  Not they aren't worth it but they were going to be about 15" deep.   My room was only 19'6" deep.  I would have lost another 2'6" with a room depth of only 17'.  Yuck!   The exterior of the room was 22' , with 16" thick walls all around.  He cost me several months delay in time and materials.   

I use two pairs of Shakti Hallographs (2 in the front wall corners are certainly out of the way and two against the mid-walls).  I also use 34 Synergistic Research HFTs... The front wall HFTs... serve the function of 180 degree diffusers to a great extent.   I would like to add that the front 5'+ high HFT-X diffuser is the most critical to high-mid range balance and the four HFT 2.0s on the side of the speakers are the most critical for the bass vs. mid-treble balance.   The other 29 HFTs, not so much but they work together.  Hardly anyone mentioned their presence.  

I recommend quadradic diffusion if your room permits them.  Mine didn't.  

@dekay  We have two libraries of books in my home (75% my collection/25% my late former wife's).   We took a 12' wall in a spare bedroom and installed book shelving 8' high to store about 1500 books (she was an historian so 30% of those books are of California/Los Angeles/local history).  There are books on comedians, seashells (she was a collector), architecture, art, U.S. history among the topics.  Then in our formal and large family room, we have beautiful built-in bookcases on either side of our fireplace (room has a 19' peak) with another 1000 art books, Judaica, entertainment (TV/Movie) books and leather bound fiction.   Another closet is filled with at least 500+ paperback fiction (I read my fill in my youth-prefer opera now).  I know, this doesn't have much to do with audio.  Read my next post. 

For more on Acoustic Fields/Dennis Foley in buyer beware: = https://www.whatsbestforum.com/threads/buyer-beware-dennis-foley-and-acoustic-fields.33886/ 

and my personal  Feburary 24, 2022 comments/experience  https://www.whatsbestforum.com/threads/my-failed-experience-with-dennis-foley-and-acoustic-fields.34449/   

@jumia 

I also acquired/rescued 40 lawn bags of books from the estate of the late Universal  pictures conductor Joseph Gershenson which were left out in 1988 for trash.  I took the 1,000+ books, donated half to CSUN (many were 1st editions/unsigned) who sold them at their book sale fundraiser.  My late former wife created the Friends of the Library there so it was a very appropriate donation.  I kept the music and history books. 

Glad you kept the Sandburg books.  For my late former wife's last birthday, I purchased the only remaining seven years (1916-1923) of the Owensmouth Gazette, bound by the former owner, Edgar Rice Burroughs (Tarzan author) who also founded and lived in Tarzana, CA.  Since my late former wife was the historian of the San Fernando Valley (she had three urban archives exhibits, one on line by the Getty Museum), president 10 years of the SFV historical society, this was the greatest gift I could give her.  Unfortunately, she died (after an 11 year systemic lupus illness) a few months later.  I then had the Huntington Library make a negative and positive microfilm copy which I donated to CSUN.   

Unfortunately, university libraries have greatly diminished book collections, at least not on the shelves, whose spaces are now occupied by banks of computers.

@dekay  We are neighbors!  Bobbette Fleschler & Catherine Mulholland founded the Friends in 1993.  I know because when I made the final bid for my house she was at the founders meeting and was elected first president that day.  (We got the house because of the photo).  Bobbette was in the Daily News newspaper in a large photo on the 3rd page showing her with our young daughter in the CSUN orange grove.  She was partly responsible for saving it (and was featured on a Huell Howser show).  You can appreciate my final gift to her.