The classic textbook on type design is 'The Elements of Typographic Style
' by Robert Bringhurst. It is used in several type design courses. I found it to be a very dry read and heavy going.
If you can get hold of a copy then 'Letters of Credit' by Walter Tracy is a good read. However it is out of print and any copy you do find will probably be expensive.
Another book which is not acclaimed as a classic but perhaps ought to be is 'Designing Type
' by Karen Cheng. This is a large book with lots of illustrations. I found it to be very easy to read and very understandable. It goes into the design of each individual letter in the Latin
alphabet in great detail and how they relate to each other.
If you are interested in other languages then there is an interesting book called 'Bi-Scriptual
' by various authors. This book is not about type design. Instead it is about various different languages and how they relate to the latin script from a graphic design perspective. However this book is quite useful if you wish to design a typeface which has characters in one of these scripts. Some of the authors do talk about what is a good and bad design of characters for their language. Each chapter covers a different language and is written by a graphic designer who is a native speaker of that language.
The languages covered are Arabic
by Lara Captan & Kristian Sarkis, Cyrillic
by Eugene Yukechev, Devanagari
by Vaibhav Singh, Greek
by Gerry Leonaidas, Hangul
(Korean) by Jeongmin Kwon, Hanzi
by Keith Tam, Hebrew
by Lirion Levi Turkenich & Adi Stern and Kanji/Hiragana/Katakana
(Chinese and Japanese) by Mariko Takagi.
Although this last book is not about type design it does provide much useful background information when designing characters for the languages covered.
Hope this helps.