According to a July 21 report by Susquehanna Financial Group’s analyst Marianne Wolk, the estimated numbers of Apple iPad owners exceed the number of Amazon Kindle owners; thereby indicating that Amazon. com is facing increased e-book pressure from Apple.

Noting that, with the iPad’s estimated ownership base having supposedly surpassed that of the Kindle, Amazon’s leverage with the publishers may be adversely affected, Wolk, putting forth the ownership statistics, said: “Last night, Apple stated it has shipped 3.27 million iPads since the April product launch, surpassing our estimate for an installed base of around 3 million Amazon Kindles to date.”

Nonetheless, Wolk also added that the sales of the Kindle devices have accelerated of late, “with the growth rate of Kindle units up 3x since the June 21 price reduction from $259 to $189.”

Wolk also said that an advantage that the Kindle has over the iPad is its early lead in e-book sales, along with the more compelling overall scope of its offerings. Statistically speaking, Amazon offers over 630,000 e-books to readers, vis-à-vis Apple’s much narrower title base of 75,000.

At the same time, Wolk also noted that Apple currently faces iPad supply constraints, which when ease can boost the device’s shipments to between 12 and 15 million units this year, thereby making the iPad an increasingly attractive option for the publishers.