The other day with a hook order I received a copy of the booklet; "How to Choose and Use Fly-tying Thread", by Paul C. Marriner. It is a booklet put out by UNI and sells for $3.00 although I got mine for free .

It has a description of different thread types and construction, bobbins and really good descriptions and photos of different thread techniques. I found it to be worthwhile and maybe some of the beginning to intermediate tiers might as well, especially for free or $3.00. I'm sure lots of this stuff is covered in newer tying books but I haven't bought a how-to fly tying book since Indian necks were the pinnacle.

I got mine from Lines End but I'm sure lots of UNI products dealers have access.
[url=http://www.linesend.com/:f7404]http://www.linesend.com/[/url:f7404]