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The following is personal observation, so not reliably sourced, but may prompt research from someone better placed.
Having an interest in Nature since childhood, I acquired a number of Observers Books, and subsequently other publications including a number published by Frederick Warne & Co. I have noticed that the Observer's Books volumes on a given nature topic are usually abridgements of more comprehensive volumes previously published by Warne in their 'Wayside and Woodland' series, with greatly edited-down texts and of illustrations extensively drawn from the 'parent' W&W titles. The latter were evidently aimed at an adult-level readership and some were originally published, by Warne or other publishers, as early as the 1890s. 90.200.40.9 (talk) 11:27, 4 February 2021 (UTC)[reply]