Intel Corp.; ISBN 1-55512-022-9; 1986;
Part I. Applications Programming, data types, memory model, instruction set. Part II. Systems Programming, architecture, memory management, protection, multitasking, I/O, exceptions and interrupts, initialization, coprocessing and multiprocessing. Part III. Compatibility (with earlier x86 machines). Part IV. Instruction Set.
Intel Corp.; ISBN 1-55512-023-7; 1987.
This explains the 386 features for operating system writers. It includes a chapter on Unix implementation. A lot of the 80386 architecture seems to have been designed with Multics in mind; the features are not used by DOS or by Unix.
Crawford, John H & Gelsinger, Patrick P.; Sybex; ISBN 0-89588-381-3; 774pp.; $26.95 (USA).
This is the book the Jolitzes used when they ported BSD to the 386 architecture.
Intel Corp.; 1993; ISBN 1-55512-195-0;
Pretty much the Pentium version of the 80386 Programmer's manual listed above.