A selection of letters that give a good introduction to Darwin’s scientific work (we may change these from time to time – there are a lot to choose from):
Letter 729: Darwin to Joseph Hooker, [11 Jan 1844]
Why is it important? Darwin for the first time lets his friend J. D. Hooker in on his theory ‘that species are not (it is like confessing a murder) immutable’. In this letter Darwin discusses his approach to developing a scientific theory, as well as noting the crucial difference between his theory and that of Lamarck, the mechanism for change. This letter gives insight into Darwin’s ideas on proper scientific method.
Letter 914: Joseph Hooker to Darwin, [4–9 Sept 1845]
Why is it important? In this letter, Hooker dismisses a work of Frédéric Gérard because Gérard ‘does not know what it is to be a specific Naturalist himself’. Hooker discusses the nature of a ‘specific’ character (i. e. one that can be used to determine whether a specimen belongs to a species).
Letter 915: Darwin to Joseph Hooker, [10 Sept 1845]
Why is it important? Darwin’s response to Hooker’s letter of [4–9 Sept 1845] reveals him to be in agreement with Hooker on the necessity of having experience in taxonomic work in order to be able to theorise about the nature of species.
Letter 1012: Darwin to Joseph Hooker, [26 Oct 1846]
Why is it important? Darwin planned to write a paper on the anomalous barnacle he had discovered on the Beagle voyage. He asked Hooker to assist him with accurate drawings of this microscopic creature. This letter illustrates the importance of having a ‘trained eye’ as well as the difficulty of describing a zoological specimen as unique as this barnacle. Darwin had to invent new terminology to describe the morphological features of barnacles and, instead of a paper, eventually wrote a multi volume monograph on the whole sub-class Cirripedia.
Letter 2136: Darwin to Asa Gray, 5 Sept 
Why is it important? Darwin tells Gray that the ‘facts’ of adaptation were what kept him ‘scientifically orthodox’ for a long time. He then reveals how he believes he has surmounted the difficulty with his theory of natural selection. What this letter highlights is how Darwin was able to turn on its head the old notion of adaptation as a state of being and give it new meaning as a process which resulted in changing forms of life over time. The modern science of ecology is underpinned by this Darwinian concept of adaptation as dynamic rather than static equilibrium.
Letter 2271: Darwin to W. B. Tegetmeier, 9 May 
Why is it important? Darwin was seeking evidence to support his view that the hexagonal form of the honey bee cell could be accounted for without assuming either advanced mathematical reasoning by bees themselves, or the direct intervention of a creator. Darwin’s approach to the problem was to observe the process of hive building. He already had an idea that the nascent cells were not hexagonal, but wanted independent confirmation of his observations.
Letter 2649: Darwin to Thomas Huxley, 11 Jan 
Why is it important? Much of Darwin’s experimental work, particulary on plants, investigated the causes of sterility or imperfect fertility. Darwin studied both self and hybrid sterility in great depth in order to work out whether sterility could be ‘selected for’ and to understand the nature of species and varieties.
Letter 4337: F. H. G. Hildebrand to Darwin, 10 Nov 1863
Why is it important? Hildebrand was inspired by Darwin’s work on orchids and on dimorphism to repeat and extend some of Darwin’s experiments, generally confirming, but in some cases challenging Darwin’s results. Darwin worked hard to get a paper by Hildebrand reprinted in English. The letter illustrates how Darwin encouraged other researchers to investigate similar topics or work with the same experimental subjects to extend and modify Darwin’s own work.
Letter 12745: Darwin to K. E. S. Wedgwood, 8 Oct 
Why is it important? Darwin had observed the cumulatively great effect of the action of worms in turning over the soil, but he was aware that although these effects could be seen in many areas, there might be some conditions which would not be conducive to worms. Here he suggests a simple way to test his hypothesis.