Borsley, Robert D. 1991.
Syntactic Theory: A Unified Approach, London: Edward Arnold.
[This book provides an introduction to Phrase Structure Grammar (including HPSG) and GB
together, comparing accounts of both approaches as different linguistic
phenomena (e.g. Passivization, Control etc.) are introduced. Thus, it is a good
source for comparing the two frameworks.
Borsley, Robert D. 1996.
Modern Phrase Structure Grammar, Cambridge, MA: Blackwell
Publishers Ltd.
[This is laid out similarly to Borsley (1991) above, but instead of GB it
introduces HPSG together with GPSG.
Carnie, Andrew. 2002.
Syntax: A Generative Introduction, Oxford: Blackwell Publishing.
[This book is mainly dedicated to the Chomskyan approaches to
syntax, but it
provides a chapter on HPSG. It provides a concise overview. It is not the best
source for precision and detail, but I found it one of the most accessible
introductions.
Cooper, Richard P. 1996. Head-Driven Phrase Structure Grammar. In K. Brown and J. Miller (eds.), Concise Encyclopedia of Syntactic Theories, 191-196. Oxford: Pergamon. [Another concise introduction with a little more mathematical detail than Carnie (2002), but still accessible.]
Partee, Barbara, Alice ter Meulen & Robert Wall. 1990. Mathematical Methods in Linguistics, Dordrecht: Kluwer Academic. [This should help those who have little mathematical background.]
Pollard, Carl and Ivan A. Sag. 1994. Head-Driven Phrase Structure
Grammar, Chicago and Stanford: University of Chicago Press and CSLI
Publications. [Often recommended in the literature.
Pollard, Carl. 1997. Lectures on the Foundations of HPSG. Unpublished manuscript: Ohio State University. <http://www-csli.stanford.edu/~sag/L221a/cp-lec-notes.pdf> [This provides a comparison between Chomskyan approaches and HPSG (mainly advocating the advantages of HPSG). The first part is a good introduction to the way HPSG views syntax, without getting into mathematical detail. The second is a good introduction to the formalism.]
Riehemann, Susanne. 1995. The HPSG Formalism. Unpublished manuscript: Stanford University. <http://www-csli.stanford.edu/~sag/L221a/hand2-formal.pdf> [A good mathematical introduction to the formalism.]
Sag, Ivan and Thomas Wasow. 1999. Syntactic Theory: A Formal
Introduction, Stanford: CSLI Publications. [This
textbook is often recommended as a very accessible introduction. There are
instructor’s notes for
this book
available online which could serve as a guide for those learning HPSG on their
own.]
http://www.ling.ohio-state.edu/research/hpsg/
[The Ohio Website. One of the main online points of reference (together
with the Stanford site). It provides a good point of reference for recent
research. It will probably be more useful after you acquire some background in
HPSG.]
http://hpsg.stanford.edu/ [The Stanford HPSG site. As above, a great point of reference that leads
to research that should become more accessible after doing some background
reading.]
http://www-csli.stanford.edu/~sag/L221a/2002.html
[This is the course profile of a HPSG course taught by Ivan
Sag in Stanford. It contains links to introductory notes used in the course.
I found this site very helpful as a beginner.