There are a lot of books on Classical design, sadly the great majority of books on the market today are more of the coffee table variety than texts giving useful instruction. Useful books fall into four categories.
1. Architectural Treatise
This, to me, is the most important category (as you may have noticed from reading through articles on the rest of the website). These are instruction books that have, from time to time, been written by architects throughout history. They are effectively the 'How to' books of their time and were used extensively by the large portion of the population that couldn't afford the services of a professional architect. They also effectively disseminated the fashions of London (or wherever the author was from) around the western world.
William Pain - The Practical builder (1774)
This book is first on the list because if you're new to classical design and just want an 18th century book that will give you classical details and tell you what size they should be then this is the easiest.
Pain is not one of the great 'authorities' and so should be used as a source of details such as cornices and architraves rather than as a book that will greatly expand ones knowledge of classical architecture.
Available on Amazon.
Sir William Chambers - A treatise on the decorative part of civil architecture (1759)
Probably my favourite architectural treatise mainly because it is so well written. Chambers was George III's architecture tutor and his style of writing is very much in the style of a teacher explaining to his pupil why things should be a certain way.
Another interesting fact about this book is that it was used by architecture schools right up until the late 19th century. If you are serious about classical design then this is probably the book that you should be reading.
(Note: There are many different editions of this book. While the original is very good I recommend the 1825 version edited by Joseph Gwilt as it contains many extra and very useful annotations. However I also recommend skipping over Gwilt's analysis of Grecian architecture until you are familiar the general concepts of classical architecture.) Available on Amazon.
Andrea Palladio - Four books of architecture. (1570)
Probably the most influential book about architecture ever written. The book spawned the hugely influential Palladian movement which dominated early 18th century architecture in Britain and also spread across to America.
From a practical point of view, the first book is an excellent introduction to the classical way of thinking and his orders are authoritative. Caution should be followed in sticking too closely to Palladio's advice when designing for today as some aspects of his designs are based on what would suit the environment that he was working with; namely Italy, and so are not appropriate for colder, darker, northern climates. Later, 18th century, architects such as Chambers address these needs better. Available on Amazon.
James Gibbs - Rules for drawing several parts of architecture. (1732)
If you wish to learn how to draw the classical orders of architecture then 'Rules for drawing' is the obvious starting point. Gibbs' book is not the most academic of the Treatise but when I was first learning to draw the orders I found that this was an advantage as it prevented 'information overload'.
Gibbs was hugely influential in his day and his other book (see below) 'A book of architecture, containing designs......' was probably the most widely used of the 18th century. For somebody wanting to learn how to design this book is more useful. When reading this book I also find it reassuring to think of all the wonderful buildings that Gibbs produced that are around today.
This book is effectively out of print, if you know of a retailer who is selling this book (and it's a good quality edition) please let us know so that we can recommend them.
2. Modern Treatise
Books written (comparatively) recently that illustrate the concepts of classical design.
Marianne Cusato - Get your house right. (2011)
This isn't strictly speaking a treatise but it is a sort of 'How to' of classical design. Well written and very easy to follow Cusato has broken down some of the most important concepts of house design into really easy to follow sections. A lot of people struggle with the 18th century texts and just want a straight forward guide to get their house to look right; this answers those needs. Available on Amazon.
William R.Ware - The American Vignola. (1905)
I know 1905 isn't exactly modern but it's in this section because Ware's book is easy reading (a good thing) and because his take has a somewhat late Victorian/ early 20th century feel about it which is somewhat evolved from the more pure classicism of the 18th century. This is not necessarily a bad thing and the book is another great introduction to the concepts of Classical architecture. Available on Amazon.
Robert Chitham - The Classical Orders of Architecture. (1985)
Chitham's book is an attempt, somewhat successfully, to present a modern way of setting out the classical orders. While the book primarily focuses on this very specific part of classical architecture it is none the less extremely useful because the classical orders are the foundation of all true traditional design. Available on Amazon.
3. Books of architectural designs
These books contains plates (illustrations) of designs by Classical architects. I have put these in the list of useful reading because one can only really become proficient in the field of design by observation and measurement. By this I mean if you look at a design ask 'Why is it good?' Using these plates one can look at, for example, the windows on a house that is appealing and measure the proportions (width to height), measure the comparative heights of the windows on different floor, distance between windows etc....... By continually measuring one is able to start gain a knowledge of what they feels works which is important in all design both classical and modern!
James Gibbs - A book of architecture (1728)
Probably the most influential book of designs in the 18th century. This book was widely used by builders not just in the British Isles but also in America and the West Indies. Available on Amazon.
Colen Campbell - Vitruvius Britannicus (First volume published 1715)
Colen Campbell was a key figure in the founding of the Palladian movement in the UK. This book of designs showcase not only his own work but also that of other contemporary and earlier architects, notably the great Inigo Jones.
Later volumes of designs were published under the same title by John Woolfe and James Gandon. For those designing on a smaller scale (non palatial!) these may be of more use. Available on Amazon.
Robert and James Adam - The works in architecture of.... (First Volume 1773)
Robert Adam was arguably the dominant force in late 18th century design. A key figure in the Neoclassical movement that adhered less strictly to Roman architectural rules than the Palladians had done. He was, however, heavily influenced by his own observations during his Grand Tour (18th century version of a Gap Year normally taken around Italy.)
Available on Amazon.
I'm not going to talk about any books here because everyone should find there own sources of inspiration. What I will say is that if you want to a competent designer then forage deeply for books, pictures and take photographs of things that inspire you. Good design doesn't take place in a vacuum, every architect and designer in history has taken inspiration from somewhere!