The standout book for me, with a lot of material I hadn't really considered before, was the second, Danger to Elizabeth, which looked mainly at the relationship between Elizabeth and English Catholics, and gives quite a substantial and detailed description of Catholic operations inside England - largely a matter of attempting to service the spiritual needs of the recusant community, though of course often tangled up with the high politics of attempted regime change and foreign sponsorship. Plowden makes what seems to me an honest effort to disentangle these strands and to tell it from the perspective of both Catholics and the government; it also of course is a very important element of the Elizabethan approach to Ireland, where religion became integral to the conflict during Elizabeth's reign in a way that had not been the case before.
The first and fourth books, Young Elizabeth and Elizabeth Regina, cover the start and end of Elizabeth's life; decent enough retellings, but I've read better elsewhere, and I was a bit shocked that Plowden assigns some blame to Elizabeth for the abuse she suffered from her stepfather as a teenager. I guess attitudes were different in 1970.
The third book, Marriage with My Kingdom, is an interesting example of writing about something that never happened, Elizabeth's marriage. Plowden takes a decently comprehensive approach to the various suitors proposed for her, starting from her childhood as a marriageable princess (or alternatively a bastard daughter of Henry VIII depending on the year) and going right through to the Duke of Alençon in her late 40s. As noted above, Plowden doesn't quite take on board the importance of Elizabeth's teenage experiences in shaping her attitude toward sexuality and relationships, preferring to concentrate on what had happened to her mother and sister (one executed, the other trapped into a loveless and unsuccessful dynastic match). Sexuality is a complex thing and one can hardly blame Elizabeth for rejecting the narrow options which were made available to her if they were not right for her.
I also found myself wondering was why no foreign Protestant suitors ever got a look-in - Eric XIV of Sweden seems to have been a serious contender at one point, but Plowden minimises this, and there were surely other eligible Scandinavian, German or Central European princes who were the right age and religion. Yet it seems to have been only Hapsburgs or French princes who were under consideration. (And Robert Dudley of course.)
Anyway, good background reading, particularly the second book.