Paul Russell 
An Introduction to the Celtic Languages [EPUB ebook] 

This text provides a single-volume, single-author general introduction to the Celtic languages.
The first half of the book considers the historical background of the language group as a whole. There follows a discussion of the two main sub-groups of Celtic, Goidelic (comprising Irish, Scottish, Gaelic and Manx) and Brittonic (Welsh, Cornish and Breton) together with a detailed survey of one representative from each group, Irish and Welsh.
The second half considers a range of linguistic features which are often regarded as characteristic of Celtic: spelling systems, mutations, verbal nouns and word order.