I don’t think too many future predictors are .300 hitters, and one ground ball I tapped out to shortstop was my hunch that the iPad wouldn’t have an immediate significant impact on ebook sales (although I thought it would be important over time.) According to data and analysis uniquely developed and provided by Michael Cader, published last Wednesday (which you need to subscribe to Publishers Marketplace to get and, if you don’t yet, what are you waiting for?), I was proved wrong in less than a month. Apparently if we get slightly larger and portable screens into people’s hands, they want to read books on them. And they don’t need to be e-ink and be lightweight (like Kindle and Nook and Sony Reader and the new Kobo Reader and a slew of forthcoming devices) to have that impact.

All we know from Apple is that they sold about a million iPads in the month of April, with 3G sales beginning only at month end. (Virtually everything sold in April was wifi-only.) We got download numbers, but no real guidance about what they meant in terms of sales. We can figure out that any sales numbers we can gather are for an average installed base of 500,000 iPads.

We wouldn’t expect the monthly sales rate of a million units to be sustained; there were a lot of pre-orders and launch-hype sales in April’s numbers. But with May being launch month for the 3G version and both the wifi and 3G models available going forward, and the 3G model apparently much more popular than the wifi-only, a sale of 500,000 in May which is 3G launch month and a “run rate” of 300,000 a month going forward would seem a modest expectation. If that’s right, then the average installed base in May will be 1.25 million, in June 1.55 million. So the installed base for June will be triple what it was in April.

Cader got anonymized information from an unknown number of large Agency publishers for the April sales. He says that for most of the companies he surveyed, iBooks sales were 12 to 15 percent of their ebook total before the 3G models landed! And then two companies reported sales jumps of 300 and 400 percent on the weekend that they did. And one publisher who showed Cader figures by title revealed that there were already books on which the iPad sales exceeded Kindle sales.

Cader’s analysis pointed out two nuances that need to be considered when interpreting these numbers. The Agency Five impact is overstated because of relatively restricted competition. They have far fewer titles competing with them in the iBooks environment than they do in the Kindle store, the Kobo store, the Sony store, or from the ebook independents. Giant Random House and lots of smaller publishers just weren’t there. So even if the sales of all five publishers were 12 percent of their total ebook sales in April, it wouldn’t suggest that iBooks constitute that portion of overall ebook sales. Yet.

But, at the same time, these numbers also understate the impact of the iPad because iPad owners also buy and consume books on the device from the Kindle and Kobo and B&N readers which wouldn’t be reflected in Cader’s survey numbers. One ebook retailer who shares information told me that sales for his company were very strong in April. I had asked that question to probe whether sales were adversely affected by the price increases mandated by the Agency model. Were they reducing business? No, definitely not. (This is a very big sub-point, but we’ll leave it for another day.) So while one must assume that some of the sales being made from iBooks would otherwise have been made by Kindle or Kobo or another existing retailer, the market is apparently growing fast enough to mask the impact of any cannibalization.

With five of the Big Six and most of the big titles in the iBooks store, it would seem reasonable to assume that 65% of the sales potential is reflected in those books. Applying that assumption to the average of the reported 12-to-15 percent market share (13.5%) would suggest that the overall share of iBooks sales is just a tad under nine percent.

But it would seem to me that number will more than double in May. The installed base will be more than twice as high and the 3G model, from which publishers are reporting much more activity, will constitute a significant portion of the May base after having been non-existent in April. In fact, it seems at least as likely that the number could triple! So by June, we could well be seeing a quarter or more of all ebook sales occurring through iBooks. The rise will probably be slower after that (May sales will reflect the huge installed base increases generated by initial sales in April of the wifi model and in May of the 3G) but Apple climbing into a solid second place behind Kindle in 60 days is pretty dramatic.

Even more exciting for publishers is the evidence that the iBooks sales are expanding the ebook market. Cader reported that many strong titles skewed to a younger and male demographic and that iBooks sales boosted the performance of some nonfiction titles. Most people figured that the iPad would appeal to an audience of not-as-heavy book buyers compared to Kindle, which was part of the reasoning behind my own flawed expectation that sales would be modest at first. But what we may be seeing is that people who get a decent reader in their hands might consume more books digitally than they had in print. If that proves to be true, it would be very good for publishers and authors.

Meanwhile, even before this analysis was delivered, we got news last week from two publishers that increased ebook sales were their best financial news. Both Simon & Schuster and Harlequin reported that print results were disappointing, but digital sales were stronger than expected.

It was only about six weeks ago that I looked at the IDPF’s most recent numbers, applied them to what I’d heard in my own anecdotal conversations with major publishers and agents, and had an epiphanic moment realizing how close we were to what we called at BISG’s Making Information Pay conference last week a “point of no return.” I wrote in my London posts and then repeated at the conference last week that I saw ebook sales to be 25% of a narrative book’s unit sales expectation by the end of 2012. With print book sales made online thrown in, I saw virtual cash registers ringing up half the units for narrative books by then. Two Big Six CEOs privately agreed with me as did a retailer knowledgable about both print and ebook sales. Then I spoke to a Big Six digital strategist who said I was being conservative.

This view is not universally accepted. An executive at a trade book distributor last week told me (nicely, he’s a nice person) that he thought I was nuts. He still sees ebook sales as trivial and not likely to reach the levels I expect by the end of 2012 by even the end of 2016.

Well, I intended to be conservative because I was so surprised at my own realization at the beginning of April. But I remind myself (and all of you) that things happen “gradually, then suddenly.” It now looks to me like the iPad — joined as it will be by a flood of new ereaders and tablets and even whole new platforms like Blio and Copia — may be the catalyst for the transition encapsuled in those three words.

When I examined the Random House tactic of staying out of the iBook store initially, I said it made sense but that it constituted a bet that iBooks sales wouldn’t be robust right out of the box. Now that sales results seem to have proven that conjecture (which I shared) wrong, I’d expect that Random House will join the other big publishers in moving to the Agency model to enable them to join the iBook offering. The numbers we discuss in this piece would suggest they’re losing sales and the agents representing the authors not in the iBooks store are bound to be pointing that out. In the meantime, Random House has gained some benefits from having less expensive ebooks in the marketplace in other storefronts, but it would be surprising if that compensated for not having an outlet selling 12% or more of the ebook units.