There are a few free reputable resources out there too. The Music Publishers Association (MPA) has a booklet titled Standard Music Notation Practice available as a free PDF. The Major Orchestra Librarians’ Association (MOLA) also has some good info. Their Guidelines for Music Preparation is useful if you want to know what is expected when doing orchestral work, and it has been translated into a bunch of languages. Some of their other resources are also helpful as well.
Most of us are probably aware of the following, but the big 4 books on engraving conventions IMO are Behind Bars by Elaine Gould, The Art of Music Engraving and Processing by Ted Ross, Music Notation by Gardner Read, and Music Notation in the Twentieth Century by Kurt Stone. Gould is probably the current “state of the art” but she doesn’t address jazz, commercial, or musical theater conventions at all, so just be prepared if that’s the type of work you’re interested in. The Ted Ross book is out of print but available on CD from NPC Imaging. For jazz and commerical conventions, Clinton Roemer’s The Art of Music Copying is excellent, but long out of print so you’re probably stuck with eBay or used book sellers.