But it is not self-evident who or what is doing the blessing. If the dedicated words are simply those which describe and praise the youth, in any book, then any and every book which contains such material is blessed by possessing it. The lines however are not easy to interpret. of their fair subject - 'to their fair subject' seems to make better sense. Of their fair subject, blessing every book. If the latter meaning is intended, then fair subject of the next line could encompass any subject or theme of which the author wrote, and not refer to the youth himself. The dedicated words which writers use The dedicated words = words dedicated to portraying (your) beauty words used in a dedicatory preface, as for example that of Shakespeare to the Earl of Southampton in Venus and Adonis and The Rape of Lucrece. In connection with the marriage obligations which are said not to apply here, there is a suggestion of a sexual peccadillo, that of eyeing a woman up and down to assess the enjoyment of having her as a sexual partner. The term is a legal one derived from the verb 'to attain' (see OED attain.v.3.). And therefore mayst without attaint o'erlook without attaint = without being convicted of sin or crime. The youth may look on the seductive words of other love poems and be tempted by them, without loss of faith. and therefore - the implication is that no breach of marriage vows is involved. I grant thou wert not married to my Muse, thou wert not married.

Hammer time 1.8.9