In the first half of the last century there was a changing of the guard both by collectors and dealers, and by the 1930's, specialization was a trend that was clearly established. In Boston, Goodspeed's was the established history and genealogy shop and an important source of material, while in New York, dealers such as Edward Eberstadt, Peter Decker, Lathrop Harper, Charles Heartman and Argosy found success providing rare materials to Thomas W. Streeter and others who were building important collections of Americana. In Philadelphia there was Dr. A. S. W. Rosenbach. Among these men Frank Siebert appeared as a new and serious collector. In 1945, for the first time, Dr. Siebert's name appears in the ledgers of the Rosenbach Company with the purchase of David Cusick's Sketches of Ancient History of the Six Nations, the rare second edition printed in 1828, for which he paid $135.* His final purchase, from the Rosenbachs in 1953, was a two-page letter from Andrew Jackson to William B. Lewis for which he paid $285. These items realized $4,025 and $46,000 respectively when sold in 1999 as lots 234* and 611. From the fall of 1945 through 1953, when the firm's business wound down, Dr. Siebert made forty purchases from Rosenbach. Many of these purchases were H. V. Jones' copies.

It takes nothing from other collectors to say that Dr. Siebert's approach was unique among the strongest collectors of that time in that he needed to substitute intelligence and guile for money as he had the former but never enough of the latter to satisfy his collecting ambitions. When prices were right and his funds and credit ample he simply bought all that he could so long as the book or books in question were already known to him and had earned a place on the lists he prepared and continuously edited to both identify and prioritize the material he wanted to acquire. Compare that approach to the collector today who, with only the vaguest concept of what he wants, contacts his dealer to find out what they have. In his time he was close to unique in his obsessive approach to the material he collected. Today it is clear that such a focused approach was the key to Dr. Siebert's rarest of the rare double victories in book collecting. This is not to say that other important collectors of the period did not do well, even as well. Thomas Streeter, Lilly, Bernardo Mendel, Holliday and DeGolyer all obsessively pursued their collections with exceptional success. They generally had more money to spend. Dr. Siebert's principal tool was his intellect.

Mr. Siebert did not have a personal or family fortune. His investments were substantial enough in his day that he would have been considered wealthy, although he saved his wealth for his books and apparently hid it from his family to invest in his collecting passion. He was well-to-do, a doctor and both brilliant and obsessed. He was also that rarest of collectors who, with a goal emerging early in his collecting, could understand that he would both have to employ a highly disciplined approach to his collecting and not be dependent on dealer advice. "He sought opinions only from those few experts he respected and his opinions were highly valued by others" according to Bailey Bishop who prepared the first draft and reviewed the final drafts of Sotheby's catalogues for the 1999 sales.