The original issue did far better than expected. Apparently, while both Pournelle and his publisher hoped for the best when it came out, the reception was superb.
Thence came the demand for a sequel and later, after that, for the next one. In the mid 1980s, Pournelle was asked about this at a Los Angeles SF Convention, and he replied "Soon". Indeed, the third volume was soon published. But, like any good author who knows how to write well and milk a successful series, dangling threads were left. It is from these that other reviewers here have asked for a fourth volume.
It has now been 15 years since the 3rd volume! Frustrating. Sadly, it appears that Pournelle's productivity has fallen in recent years. Still, we can only hope... And also hope for a reissue, maybe in hardcover, of THIS first volume. As you may recall, the first publication was in an extra large paperback format, with illustrations. It has been sufficiently long that perhaps now a hardcover with the lovely drawings might be justified.