I spent a while scouring some sites for free illustrated ebooks about plants and gardening, not that I came up with much that was of interest to me. One title did catch my eye .....
(right click - open in a new tab - will show a larger view of illustrations)
... so I downloaded a copy to the iPad to see what it was about. I tried both the EPUB and PDF versions. The EPUB version loads pages quickly but is spoilt by short amounts of spurious gibberish text over many of the illustrations. This will be a result of whatever automated process was trying to interpret the fancy lettering. The PDF version is a perfect copy but each page can take a while to appear.
Here are a couple of pages - Daffa-Down-Dilly and Eyebright:
These are typical of what to expect from the rest of the book. I would find it difficult to identify many of the plants mentioned from the illustrations but then it was designed as a fantasy entertainment, not as a botanical reference.
For those who like to look for ebooks on the iPad, especially free ones, a useful app is 'eBook Search', itself a free download from the app store. In the app you can browse or search Project Gutenberg's 33,000 public domain books; Feedbooks (Classics and modern original books); Baen Books (SciFi and Fantasy), Smashwords (books from independent authors) and the Internet Archive's 1.8 million titles. Some sites have a mixture of free and paid books. Once a title is chosen it can be downloaded directly to the iBook reader or many other apps which can handle eBooks. Quality of copies, especially public domain and archived books can vary a lot but with only download time to lose it is worth a look at anything which tickles your fancy.
Of course using an iPad is not the only way to find and read eBooks. There are programs for the Mac and PC which can also do the job.