39,516,981,435 books3.tar.gz -- 36.5% compression ratio

108,371,325,720 tar -- uncompressed

Recompressed with 7-zip and xz:

25,221,357,605 b3.7z # with flags: -m0=ppmd (23.3%)

27,077,329,052 books3.tar.xz # with flags: -e9 (25.0%)

To see what other slower compressors could do, I checked results from a random 1,000 books. MCM would achieve about half the original tar.gz file size, but it's very slow.

104,043,048 q-x11.mcm - 19.0% -- from q.tar

106,181,190 q-m9.mcm

114,564,722 q.tar.bsc-m03-b1000000000

118,636,713 q.tar.bsc-m03

130,621,411 q.ppmd -- 23.9%

137,874,349 q-s29.tar.lzip

138,274,566 qultra.7z

141,815,136 q.lzma

142,626,724 q9.xz -- 26.1%

146,786,015 q.brotli

146,933,603 q.p7zip

146,933,619 q.7z

149,371,106 q9.bz2

198,213,051 q.gz -- gzip -9 -- 36.3%

198,213,193 q.zip

227,074,912 q.lz4

228,463,000 q.lzop

546,201,600 q.tar