Tokuma Shoten Intermedia produced two books about programming for the PC-Engine in Japan - one for Assembly-language, and one for BASIC. The Assembler book was the first to be released, in early 1996. BASIC followed toward the end of 1996.
Each of the books includes a boot disk (for MSX and NEC PC-9801), and a Super CD-ROM for the PC-Engine. A box (the "develo box") needs to be connected between the computer and the PC-Engine, in order for the two machines to communicate. The schematic for this 2-chip wonder in actually included in both books. (!)
In August 1996, a set of patches was released for the Develo system to work on IBM-type PC's. The patches were contained on the pack-in CDROM in August 1996's PC-Engine Fan magazine. Unfortunately, the sprite and background editor was not fixed. Also, the patch files were not on an ISO-9660 filesystem (or any PC-readable filesystem). In order to retrieve them, one would have needed either: (a) a friend with a working setup, to help extract the files, or (b) to be resourceful.
Both books are written entirely in Japanese, but since they are technical manuals, there are some English words inside. Furthermore, a substantial portion of the technical terms are simply English technical terms, written using the Japanese katakana character set.
This book contains a BASIC programming language that the Develo people have built for the PC-Engine. The technical information provided is less detailed, but the "how-to" information for how to write your own games appears to be more detailed. Sample programs are included.