I am fascinated by this subject currently, for obvious reasons.
There are some good websites you can look at for 'what your baby should be doing at x months' but they tend to be quite basic.Then there are your 'Gina Ford' type books which give parenting advice but very much come from one angle and tend to be on a certain topic like establishing a routine.
At the other end of the spectrum, from what I can see, there are books on 'early years education' which are aimed more at students / early years practitioners and are perhaps too theoretical or advanced.
I am enjoying 'making' my own toys and games for Mabel, be that actually sewing a sock monster (I am on with that) or using household objects to stimulate her sense of touch. Yesterday I found that when sitting her up against my knees and dangling a bib (easy to hold) near to her hand she would actually grab it. This is a very new thing and it was mesmerising to watch her complete concentration (all be it for a few minutes max) on controlling her arm.
Perhaps it is driven by missing work (designing training) and needing some intellectual stimulation myself or my ongoing need for a bit of structure but I am wanting to learn more about how she develops and practical tips on what I can be doing to help her along the way.
I've not found the answer yet but shall be asking my very talented brother in law who is an expert on early years development (http://abcdoes.typepad.com/abc-does-a-blog/) for his advice on books and spending some time researching websites and blogs to find the best ones.
I'll update my findings soon!!!